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THE  VEGETATION  OF  A  DESERT  MOUNTAIN 

RANGE  AS  CONDITIONED  BY 

CLIMATIC  FACTORS 


BY 

FORREST  SHREVE 


WASHINGTON,  D.  C. 

Published  by  the  Carnegie  Institution  of  Washington 
1915 


_,^ 


fl* 


^^^ 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
Publication  No.  217 


Copies  of  this  Bo<^ 
were  first  issued 

OCT  2  8  1915 


PRESS  OF  J.  B.  LIPPINCOTT  COMPANY 
PHILADELPHIA 


"''  ^-  ^tate  Col/ege 


CONTENTS. 

PAOE. 

Introduction 5 

Geography  and  Topography  of  the  Santa  Catalina  Mountains 6 

Vegetation  of  the  Santa  Catalina  Mountains 11 

The  Desert  Region 15 

The  Upper  Bajadas 15 

The  Desert  Arroyos  and  Canons 19 

The  Lower  Desert  Slopes 22 

The  Upper  Desert  Slopes 23 

The  Encinal  Region 24 

The  Lower  Encinal 25 

The  Upper  Encinal 27 

The  Forest  Region 29 

The  Pine  Forest 31 

The  Fir  Forest 33 

Flora  of  the  Santa  Catalina  Mountains 36 

Phytogeographic  Relationships  of  the  Flora 36 

The  Desert  Flora 36 

The  Encinal  Flora 38 

The  Forest  Flora 39 

List  of  Characteristic  Species 41 

CUmate  of  the  Santa  Catalina  Mountains 46 

Rainfall 48 

Seasonal  Distribution  of  Rainfall 48 

Altitudinal  Increase  of  Rainfall 51 

Soil  Moisture 59 

Evaporation 63 

Humidity 67 

Temperature 69 

Length  of  Frostless  Season 70 

Normal  Altitudinal  Temperature  Gradient 75 

The  Absolute  Minimum  of  Winter 79 

Departures  from  the  Normal  Temperature  Gradient  due  to  Cold-air  Drainage  82 

Soil  Temperature 86 

Correlation  of  Vegetation  and  Climate  in  the  Santa  Catalina  Mountains 88 

The  Normal  Altitudinal  Gradient  of  Vegetation 88 

The  Vertical  Distribution  of  Individual  Species 89 

Physical  Factors  Involved  in  the  Determination  of  the  Normal  Altitudinal  Gradient 

of  Vegetation 91 

Moisture  Factors 92 

Temperature  Factors 94 

The  Role  of  Topographic  Features  in  Determining  Departures  from  the  Normal 

Altitudinal  Gradient  of  Vegetation 97 

The  Role  of  Slope  Exposure 97 

The  Role  of  Streams  and  Flood-plains 104 

The  Role  of  Topographic  Relief 107 

General  Conclusions 109 

3 


12543 


THE  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE  AS 
CONDITIONED  BY  CLIMATIC  FACTORS. 

INTRODUCTION. 

The  southern  half  of  the  state  of  Arizona  may  be  briefly  character- 
ized as  a  relatively  level  plain  studded  with  numerous  hills  and  moun- 
tains. The  plain  rises  from  elevations  of  a  few  hundred  feet  along  the 
Colorado  River  to  as  much  as  4,500  and  5,000  feet  near  the  New  Mexi- 
can boundary.  The  lower  elevations  follow  the  Gila,  Salt,  San  Pedro, 
and  other  rivers,  while  the  higher  plains  surround  the  loftier  mountains 
of  the  southeastern  portion  of  the  State.  Between  the  Colorado  River 
and  Tucson  there  are  no  mountains  of  commanding  elevation,  and  the 
area  occupied  by  the  scattered  volcanic  peaks  and  ranges  is  not  more 
than  one-tenth  of  the  total  area  of  the  region.  To  the  eastward  of 
Tucson,  however,  a  much  greater  percentage  of  the  total  area  is  occu- 
pied by  mountain  ranges,  a  score  of  which  reach  elevations  of  over  8,000 
feet.  The  general  topographic  configuration  of  the  region  has  remained 
unchanged  throughout  a  long  period  of  geological  time,  and  the  moun- 
tains and  hills  have  been  subjected  to  prolonged  erosion,  the  products 
of  which  have  served  to  build  up  the  shelving  plains  which  form  the 
intervening  valleys. 

Those  portions  of  southern  Arizona  which  lie  below  4,000  feet  are 
covered  with  a  low,  open,  desert  vegetation,  while  the  plains  and  valleys 
of  higher  elevation  support  a  loose  carpet  of  perennial  grasses  and 
ephemeral  herbs,  together  with  a  sparse  representation  of  succulent  and 
semi-succulent  types  of  plants.  The  higher  mountain  ranges  exhibit 
a  graduated  sequence  of  vegetation  from  that  of  the  desert  valleys, 
through  a  scrub  of  evergreen  oaks  to  forests  of  pine,  spruce,  and  fir. 
The  bodies  of  mesophilous  vegetation  which  occupy  these  isolated 
mountain  summits,  and  the  stages  which  connect  them  with  the  vege- 
tation of  the  desert,  present  innumerable  phenomena  of  the  greatest 
interest  to  both  physiological  and  floristic  plant  geography,  and  form 
a  most  fruitful  field  of  investigation. 

The  Santa  Catalina  Mountains  are  one  of  the  most  westerly  of  the 
high  ranges  of  southeastern  Arizona,  and  rise  from  an  approximate 
basal  elevation  of  3,000  feet  to  a  height  of  9,150  feet.  With  respect  to 
their  vegetation  these  mountains  are  typical  of  a  large  number,  not 
only  in  Arizona  but  in  southern  New  Mexico  and  northern  Mexico  as 
well.  Their  location  within  20  miles  of  Tucson  and  their  ready  acces- 
sibility from  the  Desert  Laboratory  have  given  opportunity  for  a  study 
of  the  distribution  of  their  vegetation  and  for  a  measurement  of  some 
of  the  physical  factors  upon  which  the  existence  and  activities  of  the 
vegetation  depend.  It  is  the  purpose  of  the  present  paper  to  give  a 
brief  description  of  the  vegetistic  features  of  the  various  altitudes  and 


6  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

topographic  situations  in  the  Santa  Catalina  Mountains,  to  give  the 
results  of  the  chmatological  instrumentation  which  has  been  carried 
on,  and  to  indicate  in  so  far  as  possible  the  manner  and  degree  in  which 
the  successive  altitudinal  stages  of  vegetation  are  dependent  upon  the 
gradients  of  climatic  change  by  which  they  are  accompanied. 

Some  of  the  instrumental  records  date  from  the  summer  of  1907, 
the  first  year  in  which  members  of  the  Desert  Laboratory  became 
interested  in  the  mountains,  but  the  principal  part  of  the  data  to  be 
presented  were  secured  in  1911  and  subsequent  years.  The  operation 
of  instruments  and  the  study  of  vegetation  have  been  made  on  visits 
of  5  to  10  days,  at  intervals  between  April  and  October.  From  three 
to  nine  such  visits  have  been  made  to  the  mountains  in  each  of  the 
summers  since  1910. 

The  practical  exigencies  of  the  work  have  limited  the  character  of 
the  instrumentation  which  could  be  carried  out,  but  have  not  impaired 
the  accuracy  of  the  data  which  it  was  possible  to  secure.  There  is  no 
respect  in  which  the  results  herewith  presented  may  be  considered  as 
more  than  a  mere  outline  of  a  large  and  widely  ramifying  botanical 
problem.  The  central  interest  of  the  writer  has  been  to  determine 
which  of  the  major  environmental  factors  are  responsible  for  the  chief 
distributional  features  of  the  vegetation,  and  to  ascertain  something 
regarding  the  intensities  of  the  factors  responsible  for  the  distributional 
limits  of  individual  species,  and  thereby  for  the  limitation  of  the  types 
of  vegetation  themselves.  Such  an  inquiry  into  the  correlations  exist- 
ing between  physical  conditions  and  the  occurrence  and  activity  of 
plants  may  do  much  to  explain  general  vegetistic  phenomena,  but  it 
does  far  more  to  open  up  the  innumerable  physiological  problems  which 
must  be  well  known  at  the  outset  to  underlie  these  correlations. 

GEOGRAPHY  AND  TOPOGRAPHY  OF  THE  SANTA 
CATALINA  MOUNTAINS. 
The  Santa  Catahna  Mountains  occupy  the  drainage  divide  between 
the  San  Pedro  River,  a  tributary  of  the  Gila,  and  the  Santa  Cruz,  a 
river  which  seldom  has  sufficient  flow  to  reach  an  outlet  in  the  Gila. 
The  position  of  the  mountains  is  between  110°  30'  and  111°  east  longi- 
tude and  32°  15'  and  32°  35'  north  latitude.  The  general  outline  of  the 
range  is  roughly  triangular  (see  plate  40),  its  southern  base  being  at 
about  3,000  feet  (915  m.)  elevation,  its  northeastern  base  (parallel  to 
the  San  Pedro  River)  lying  at  approximately  3,500  feet  (1,065  m.). 
To  the  northwest  a  broad  grassy  plain,  3,500  to  3,800  feet  in  elevation, 
connects  the  Santa  Catahnas  with  the  lower  Tortilla  Mountains.  To 
the  southeast  a  narrow  pass,  4,300  feet  (1,310  m.)  in  elevation,  connects 
with  the  closely  adjacent  El  Rincon  Mountains,  which  reach  an  eleva- 
tion of  8,465  feet  (2,580  m.).  Southward  from  El  Rincon  range  a  pass 
of  4,000  feet  (1,220  m.)  elevation  leads  to  the  Santa  Rita  range  (9,432 


GEOGRAPHY  AND  TOPOGRAPHY  OF  SANTA  CATALINA  MOUNTAINS.        7 

feet,  2,875  m.).  To  the  northeast  of  the  San  Pedro  River  rises  the 
GaUuro  range  of  mountains,  the  main  ridge  of  which  is  approximately 
35  miles  (57  km.)  distant  from  the  Santa  Catalinas.  The  next  moun- 
tains encountered  in  passing  northeastward  are  the  Pinaleno  or  Graham 
range,  about  60  miles  distant  from  the  Santa  Catahnas,  and  exceeding 
them  in  altitude  by  about  1,400  feet  (427  m.).  Beyond  the  upper 
course  of  the  Gila  River  lie  the  Gila  Mountains,  and  still  further  to  the 
northeast  the  White  Mountains,  which  reach  an  elevation  of  11,280 
feet  (3,440  m.)  in  Escudilla  Peak.  The  White  Mountains  present  one 
of  the  largest  elevated  land  masses  of  the  State,  connecting  to  the 
northeast,  through  the  Mogollon  Mesa,  with  the  elevated  region  which 
surrounds  the  San  Francisco  Peaks  and  supporting  a  heavy  body  of 
forest  which  extends  from  the  New  Mexican  boundary  nearly  to  the 
Grand  Canon.  East  of  the  White  Mountains  the  elevated  country  ex- 
tends for  about  75  miles  (121  km.)  into  New  Mexico,  breaking  up  into 
several  diverging  ranges  which  form  a  part  of  the  Continental  Divide, 
draining  westward  into  the  Gila  and  eastward  into  the  Rio  Grande. 

The  Santa  Catahna  Mountains  are  thus  seen  to  stand  at  the  south- 
western terminus  of  a  series  of  isolated  elevations  which  stretch  away 
from  the  southern  edge  of  the  Colorado  Plateau.  The  valley  of  the 
Rio  Grande  and  its  tributaries,  several  undrained  basins,  and  the 
valley  of  the  Little  Colorado  combine  to  separate  the  entire  chain  of 
elevations — San  Francisco  Mountains,  Mogollon  Mesa,  White  Moun- 
tains, and  the  mountains  of  western  New  Mexico — from  the  Sangre 
de  Cristo,  San  Juan,  and  Jemez  mountains  of  northern  New  Mexico, 
which  are  virtually  a  part  of  the  Rocky  Mountain  system.  A  consider- 
able degree  of  isolation  from  the  north  is  thus  given  to  the  entire  series 
of  mountains  in  southeastern  Arizona. 

To  the  south  and  southeast  of  the  Santa  Catalinas  an  irregular  but 
close-set  series  of  mountains  gives  them  a  connection  with  the  Mexican 
Cordillera  which  is  much  closer  than  their  connection  with  the  Rocky 
Mountains.  To  the  west  the  nearest  forest-clad  elevations  are  the  San 
Jacinto  and  San  Bernardino  Mountains  of  southern  California,  which 
are  about  300  miles  (480  km.)  distant. 

The  relative  isolation  of  the  Santa  Catalina  Mountains  and  the 
directions  in  which  they  possess  easy  stages  of  connection  with  other 
elevated  regions  are  of  first  importance  in  relation  to  the  genesis  and 
history  of  their  flora,  a  subject  which  will  be  only  briefly  touched  upon 
in  this  paper  (see  p.  36). 

The  southern  face  of  the  Santa  Catalinas,  to  which  the  present  in- 
vestigation has  been  confined,  is  built  solely  of  gneiss  of  varying  degrees 
of  hardness.  The  main  ridge  and  the  northern  and  eastern  lateral 
ridges  are  worn  into  a  relatively  rounded  topography,  while  the  south- 
western corner  of  the  range  possesses  rock  of  greater  durabihty  and  is 
correspondingly  more  rugged  in  topography. 


8  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

The  highest  elevations  lie  between  Mount  Lemmon  (9,150  feet, 
2,790  m.)  and  Green  Mountain  (7,900  feet,  2,410  m.),  which  are  only 
7  miles  (11  km.)  apart.  Samaniego  Ridge  and  Oracle  Ridge  extend 
northward  from  the  vicinity  of  Mount  Lemmon,  falling  rapidly  in 
elevation  and  terminating  in  the  high  plain  which  lies  in  the  direction 
of  the  Tortilla  Mountains.  A  very  rugged  ridge  extends  southwest- 
ward  from  Mount  Lemmon  and  terminates  in  Pusch  Ridge.  To  the 
south  of  the  main  ridge  an  extensive  elongated  drainage  basin  has  been 
developed  which  lies  parallel  to  the  south  face  of  the  range  and  finds 
its  outlet  through  Sabino  Canon.  Several  important  streams  drain  the 
south  slopes  of  the  main  ridge  and  are  tributary  to  Sabino  Canon.  To 
the  east  of  Sabino  two  important  drainages — Bear  Canon  and  Soldier 
Caiion — drain  the  eastern  end  of  the  main  ridge  in  the  vicinity  of  Green 
Mountain,  and  west  of  Sabino  are  Pedregosa,  Ventana,  Pima,  and  other 
canons  which  rise  in  the  rugged  southwestern  portion  of  the  mountain. 
All  of  these  streams  flow  into  the  Rillito,  a  tributary  of  the  Santa  Cruz 
which  also  drains  a  portion  of  El  Rincon  range.  The  north  face  of  the 
main  ridge  between  Oracle  and  Samaniego  ridges  is  drained  by  the 
Canada  del  Oro,  which  flows  at  first  north,  then  west,  and  finally  south- 
west, emptying  into  the  Santa  Cruz.  On  the  northeast  slopes  of  the 
range  the  topography  is  relatively  simple,  the  high  elevations  falling 
away  rapidly  in  the  direction  of  the  San  Pedro  River.  A  large  number 
of  minor  streams  drain  this  region  and  give  to  the  San  Pedro  perhaps 
less  than  one-fourth  of  the  total  run-off  of  the  mountains. 

The  main  drainageway  of  Sabino  Canon  is  the  only  one  in  the  Santa 
Catalinas  which  possesses  a  constant  flow  of  water,  which  is  due  both 
to  the  great  extent  of  its  cachment  basin  and  to  the  fact  that  it  has 
its  source  on  the  north  slopes  of  Mount  Lemmon  in  the  heaviest  body 
of  timber  on  the  mountain.  In  the  Caiiada  del  Oro,  in  Bear  and  Soldier 
Canons,  as  well  as  in  a  few  of  the  larger  canons  of  the  north  slopes, 
water  may  be  found  at  all  times  of  the  year  in  certain  locaUties  where 
the  local  configuration  of  the  valley  or  the  occurrence  of  resistant  dikes 
of  rock  forces  the  underflow  to  the  surface.  During  the  rainy  seasons 
water  may,  of  course,  be  found  in  any  of  the  large  drainageways.  The 
heavy  local  showers  of  summer  often  convert  even  the  smallest  stream- 
ways  into  rushing  torrents  for  a  few  hours. 

The  small  size  of  the  Santa  Catalinas  together  with  their  elevation 
gives  a  steep  gradient  to  all  of  the  major  streams.  The  main  stream 
of  Sabino  Caiion  falls  from  7,700  feet  at  Webber's  Cabin  to  3,700  feet 
at  the  west  end  of  Sabino  Basin,  a  distance  of  6  miles,  or  a  gradient 
of  fall  of  667  feet  per  mile.  From  the  west  end  of  the  Basin  to  its 
emergence  onto  the  desert  this  stream  falls  only  1,000  feet  in  a  distance 
of  5  miles.  The  Canada  del  Oro  falls  at  a  rate  of  494  feet  to  the  mile 
from  its  source,  just  west  of  Mount  Lemmon,  to  the  confluence  of  its 
main  tributary  from  the  west  slopes  of  the  Oracle  Ridge,  and  at  the 


GEOGRAPHY  AND  TOPOGRAPHY  OF  SANTA  CATALINA  MOUNTAINS        9 

rate  of  200  feet  to  the  mile  from  there  to  its  emergence  from  the  moun- 
tain at  3,400  feet.  The  result  of  the  passage  of  intermittent  and  tor- 
rential streams  through  such  steep  drainageways  has  been  the  wearing 
down  of  the  stream  beds  to  solid  rock  throughout  almost  the  entire 
drainage  system  of  the  mountain.  There  are  no  parks  nor  mountain 
meadows,  such  as  are  present  in  some  of  the  largest  southwestern 
mountains.  The  flood-plains  and  alluvial  bottoms  are  all  small  and 
scattered.  The  spots  in  which  meandering  streams  may  be  found  are 
very  few  indeed.  In  Bear  Canon  a  flood-plain  nearly  half  a  mile  in 
length  has  been  formed  as  a  result  of  a  large  body  of  highly  resistant 
rock,  which  has  narrowed  the  canon  and  prevented  the  outwash  of 
erosion  material.  Similarly,  in  Soldier  Canon  there  is  a  small  flood- 
plain  below  which  the  stream  falls  300  feet  in  a  very  short  distance 
through  a  narrow  gorge.  Although  Sabino  Basin  is  a  locality  in  which 
several  converging  streams  undergo  a  sudden  reduction  in  their  gradi- 
ent of  fall,  there  has  not  been  any  considerable  deposition  at  that  place. 
On  the  contrary  the  region  is  one  in  which  the  streamways  are  bordered 
and  bedded  by  large  boulders  in  a  matrix  of  coarse  sand  and  are  sub- 
jected to  active  scouring  by  the  torrential  floods  of  summer. 

Whatever  may  have  been  the  original  form  of  the  Santa  Catalinas 
they  have  been  so  far  worked  upon  by  erosion  and  weathering  that 
they  now  possess  almost  no  relatively  level  areas  or  regions  of  inde- 
terminate drainage.  All  of  the  higher  portions  of  the  main  ridge  and 
of  the  lateral  ridges  as  well  are  extremely  narrow.  The  only  localities 
in  which  the  topography  broadens  and  is  relatively  level  are  at  points 
where  several  drainages  have  their  origin,  or  places  just  above  precipi- 
tous cliffs.  On  the  sunomit  of  Mount  Lemmon  there  is  a  nearly  level 
area  of  at  least  100  acres  (see  plate  36  a  and  b),  from  which  a  flat-topped 
ridge  extends  eastward  for  half  a  mile,  terminating  in  an  abrupt  drop, 
in  the  course  of  which  two  narrow  ridges  have  their  origin.  This  re- 
stricted area  of  nearly  level  land  is  a  last  relic  of  a  portion  of  the  original 
structural  form  of  the  mountain,  and  it  will  not  be  many  centuries 
until  it  is  reduced  to  the  narrow  form  of  the  lower  ridges. 

In  the  eastern  and  central  portion  of  the  Santa  Catalinas  the  gneiss 
weathers  readily  and  gives  rise  to  a  loam  soil.  The  precipitate  topog- 
raphy gives  little  opportunity  for  the  accumulation  of  this  soil,  and 
it  is  thin  in  almost  all  localities.  Throughout  the  lower  portions  of  the 
range,  below  the  pine  forest,  the  soil  has  the  appearance  of  being  ex- 
tremely coarse  by  reason  of  the  surface  coating  of  angular  fragments 
from  1  to  5  mm.  in  diameter.  Surface  drainage  is  able  to  move  this 
material  but  slowly  by  reason  of  its  size  and  angularity.  Just  beneath 
it  may  be  found  a  fine  soil,  still  mingled  with  coarse  particles  but  held 
in  place  by  the  mulch  of  stones,  which  is  analogous  to  "desert  pave- 
ment." The  outcropping  rock  and  larger  boulders  serve  to  retard 
erosion  and  to  preserve  a  soil  sufl&ciently  deep  for  shrubs  and  trees  to 


10  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

find  root.  There  are  many  deep  soil-filled  crevices  through  which  the 
roots  of  trees  are  able  to  penetrate  to  bodies  of  deep-seated  soil  of 
favorable  moisture  content. 

The  restricted  areas  of  alluvial  soil  in  the  desert  and  lower  mountain 
regions  are  of  a  fine  sand  or  sandy  loam  and  possess  considerable  humus, 
in  contrast  to  the  soils  of  the  slopes.  At  the  forested  elevations  the 
soil  is  similar  to  that  of  the  evergreen  oak  region.  The  soil  of  the  lower 
pine  belt  is  scarcely  superior  in  depth  or  humus  content  to  that  of  the 
upper  oak  region.  Above  7,500  feet,  however,  the  amount  of  humus, 
as  well  as  the  amount  of  surface  litter,  increases  with  the  increasing 
density  of  the  stand  of  pines.  On  the  north-facing  slopes  which  are 
clothed  with  fir  forest  the  soil  is  not  much  if  at  all  deeper  than  in  the 
heavy  stands  of  pine,  but  is  notably  richer  in  organic  matter. 

The  alluvial  slopes  which  immediately  surround  the  mountain  are 
so  closely  related  to  it  in  all  of  their  physical  and  biological  features 
that  it  will  be  necessary  in  the  following  pages  to  give  some  considera- 
tion to  their  vegetation.  Throughout  the  arid  southwest  the  long 
straight  profiles  presented  by  the  outwash  slopes  of  the  hills  and  moun- 
tains form  one  of  the  characteristic  features  of  the  landscape.  The 
distinct  character  of  these  slopes  is  to  be  attributed  to  the  manner  in 
which  they  have  been  laid  down  under  conditions  of  torrential  rainfall 
and  of  violent  and  intermittent  stream  flow,  and  their  distinctness  from 
the  paraboHc  alluvial  slopes  of  the  humid  regions  has  caused  Tolman  * 
to  designate  them  technically  by  their  popular  Spanish  name ''6aja(^a."t 

The  bajadas  constitute  almost  the  total  area  of  all  the  intermontane 
valleys  of  southern  Arizona.  The  only  portions  of  the  valleys  topo- 
graphically separable  from  them  are  the  stream  beds,  the  flood-plains, 
and  the  "play as"  or  undrained  areas  into  which  one  or  more  streams 
flow  and  deposit  their  load.  To  the  student  of  vegetation  there  are 
marked  differences  between  the  upper  and  lower  portions  of  all  bajadas. 
The  differences  in  the  physical  features  presented  by  upper  and  lower 
bajadas  of  the  same  elevation  have  been  only  superficially  investigated; 
the  differences  in  their  vegetation  are  very  obvious,  as  will  be  described. 
The  principal  environmental  features  which  appear  to  differentiate  the 
high  and  low  bajadas  are  the  coarser  character  of  the  soil  in  the  high 
bajadas,  the  possibihty  of  higher  soil  moisture  in  them,  at  least  at  a 
depth  of  several  feet,  and  the  greater  development  of  calcareous  incrus- 
tations, or  ''cahche,"  in  the  soil  of  the  low  bajadas.  The  layers  of 
caliche  lie  near  the  surface  in  some  places,  while  in  others  the  upper- 
most ones  have  been  covered  by  deposition;  they  extend  downwards 
for  a  few  feet  in  some  cases,  or  more  frequently  recur  to  a  depth  of 
100  feet  or  more. 

The  bajada  of  the  southern  face  of  the  Santa  Catalinas  has  been 

*  Tolman,  C.  F.    Erosion  and  Deposition  in  the  Southern  Arizona  Bolson  Region.    Jour.  Geol., 
vol.  17,  pp.  136-163,  1909. 
t  Pronounced  bahada. 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  11 

truncated  at  its  lower  edge  by  the  Rillito,  which  flows  at  right  angles 
to  the  slope  of  the  bajada,  and  on  the  western  side  of  the  range  the 
Canada  del  Oro  has  worn  off  the  lower  edges  of  the  detrital  slopes  in 
similar  matter.  The  well-developed  bajadas  which  lie  between  Pima 
and  Ventana  Caiions  fall  at  the  grade  of  150  to  175  feet  per  mile. 
Between  Ventana  and  Bear  Cafions  the  uppermost  portion  of  the 
bajada  has  been  worn  away,  so  that  at  present  a  shallow  valley  lies 
between  the  base  of  the  mountain  and  the  lower  portion  of  the  old 
bajada,  now  cut  into  isolated  and  rounded  hills.  On  the  northeast 
side  of  the  Santa  Catalinas  the  bajada  which  extends  down  to  the  San 
Pedro  River  exhibits  approximately  the  same  grade  as  the  bajada  at 
Pima  Canon.  Its  surface  is  crossed,  however,  by  so  many  drainages 
from  the  steep  northeast  face  of  the  mountain  that  the  bajada  region 
consists  of  a  series  of  rounded  ridges  extending  out  from  the  base  of 
the  mountain,  very  unlike  the  relatively  flat  bajadas  of  the  Santa  Rita 
and  El  Rincon  ranges. 

VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS. 

The  journey  from  the  base  to  the  summit  of  the  Santa  Catalina 
Mountains  brings  to  the  eyes  of  the  observer  a  constantly  changing 
panorama  of  vegetation.  New  types  of  plants  are  constantly  being 
encountered  with  increase  of  altitude,  while  types  already  familiar  are 
being  left  behind.  There  is  no  portion  of  the  mountain,  at  least  below 
7,500  feet,  in  which  a  climb  of  500  feet  does  not  materially  alter  the 
physiognomy  of  the  surrounding  vegetation.  The  course  of  the  vege- 
tational  panorama  is  not  merely  a  gradual  transition  from  the  open 
desert  of  succulents  and  microphylls  to  the  heavy  fir  forest  which 
occupies  the  summit  of  Mount  Lemmon  (plate  1).  There  are  inter- 
posed between  these  vegetations  two  distinct  belts  of  plant  life  through 
which  this  tremendous  transition  takes  place. 

The  arborescent  cacti  and  the  trees  and  shrubs  of  the  desert  give 
way  gradually  to  evergreen  oaks,  leaf-succulents,  sclerophyllous  shrubs, 
and  perennial  grasses.  This  open  but  arborescent  vegetation  reaches 
a  full  development  and  then  gives  way  to  pine  forest,  with  a  distinctive 
accompanying  carpet  of  herbaceous  perennials.  The  pine  forest  is 
then,  in  turn,  invaded  by  spruce  and  fir  and  the  heavy  stands  of  these 
trees  are  accompanied  by  still  another  assemblage  of  shrubs  and  her- 
baceous plants.  The  striking  character  of  these  gradations  of  vegeta- 
tion is  not  due  solely  to  the  contrast  between  the  varied  vegetation 
of  the  open  desert  and  the  monotony  of  the  closed  coniferous  forests, 
but  is  quite  as  largely  due  to  the  striking  types  of  plants  which  are 
to  be  found  both  in  the  desert  and  in  the  region  of  evergreen  oaks. 

A  first  and  most  general  observation  of  these  vegetational  stages  will 
discover  the  distinctive  regions  of  desert,  of  park-like  semi-desert  and 
of  forest.  The  first  is  like  the  desert  of  the  extensive  bajada  slopes 
which  surround  the  entire  mountain:  the  second  is  similar  to  plant 


12 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


communities  which  may  be  seen  in  southern  Texas  and  in  California,  as 
well  as  in  similar  situations  in  Arizona  and  New  Mexico;  the  last  is 
essentially  hke  the  great  body  of  yellow  pine  forest  which  stretches 
from  southern  Jalisco  to  British  Columbia,  or  like  the  fir  and  Douglas 
spruce  forests  of  the  Rocky  Mountains.    These  three  major  regions 


FEET 

9,000 

8.000 
7.000 
6.000 

5,000 
4,000 
3.000 

9,000 
8,000 
7,000 
6,000 
5,000 
4^000 
3,000 

9,000 

8,000 

7,000 

6,000  - 

6,000 

4,000  H 

3,000 


Fig.  1. — Diagram  showing  vertical  distribution  of  Desert,  Encinal,  Pine 
Forest,  and  Fir  Forest  in  relation  to  slope  exposure,  together  with  dia- 
grams showing  effect  of  slope  exposure  on  vertical  distribution  of  Fou- 
quieria  splendens,  Dasylirion  wheeleri,  Quercus  emoryi,  and  Quercus 
hypoleuca. 

constitute  the  most  natural  and  easily  distinguished  subdivisions  of 
the  vegetation,  and  depend  for  their  distinctness  on  the  radical  dis- 
similarity of  the  dominant  tj^jes  of  plants  in  each.  They  may  best  be 
designated  by  the  simple  terms  Desert,  Encinal,*  and  Forest.    The 


- 

SOUTH 

^ 

NORTH 

- 

- 

^ 

^ 

^.  Fir  forest 

- 

. 

^^ 

^^ 

^m. 

. 

^^P 

^ 

^^^^s.  Pine  forest 

- 

/__ 

>..;;■;..  ■::vv\Encinai 

- 

'-, 

_/^ 

.'.-  ■.'  ;  ■.  ■  •'  ■  :  ■    •  • .  •  ■. Xpesert 

J 

■ 

/\ 

- 

- 

A 

- 

.J 

Fouquieria  splendens 

J 

Dasylirion  wheeleri 

^. 

- 

A 

- 

A 

- 

- 

^\ 

- 

/^k 

- 

. 

/^^^ 

. 

/^\ 

- 

.J 

Quercus  emoryi 

-J 

Quercus  hypoleuca 

^- 

*  The  Spanish  word  "encinal"  signifies  a  grove  or  forest  of  evergreen  oaks,  being  derived  from 
encina,  evergreen  oak.      The  suitability  of  the  word  waa  suggested  by  Prof.  J.  W.  Harshberger. 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  13 

Encinal  belt  is  essentially  a  region  dominated  by  sclerophyllous  trees 
and  shrubs  and  by  semi-succulent  perennials,  with  an  open  stand  of 
perennial  grasses.  It  is  what  is  designated  by  the  Forest  Service  as 
the  ''woodland  type"  of  forest.  The  pine  and  fir  forests  are  very 
dissimilar  in  their  floristic  composition,  but  they  are  much  more  closely 
alike  vegetistically  than  are  any  two  of  the  three  major  divisions  which 
have  been  made.  A  further  and  more  careful  examination  of  the  stages 
which  connect  the  Desert  with  the  Forest  will  discover  not  only  the 
inevitable  gradations  between  the  three  major  regions,  but  also 
several  minor  features  which  cause  constantly  recurring  departures 
from  the  typical  or  ideal  vertical  distribution  of  the  vegetation.  The 
influence  of  slope  exposure  on  the  vertical  ranges  of  both  the  individual 
species  and  the  vegetation  itself  is  a  feature  which  these  mountains 
share  with  almost  all  extra-tropical  mountains;  the  distinctive  vegeta- 
tion of  flood-plains  and  streamways  is  also  as  clearly  noticeable  here 
as  in  all  arid  and  semi-arid  regions;  the  occurrence  of  the  lowland 
species  at  higher  altitudes  on  ridges  than  in  the  valleys  is  also  a  strong 
differentiating  feature. 

In  describing  the  sahent  physiognomic  and  floristic  features  of  the 
vegetation,  and  its  distributional  behavior,  it  is  expedient  to  recognize 
primarily  the  three  major  divisions  of  Desert,  Encinal,  and  Forest, 
and  then  to  take  into  account  secondarily  the  degree  to  which  the 
components  of  these  regions  intermingle  and  the  extent  to  which  the 
topographic  irregularities  of  the  mountain  cause  an  alternation  and 
interdigitation  of  the  three  regions. 

The  basal  slopes  of  the  mountain  between  3,000  and  4,000  feet  (915 
and  1,220  m.)  present  few  vegetational  distinctions  from  the  upper 
bajadas,  and  almost  no  distinctions  of  flora.  Between  4,000  and  5,000 
feet  (1,220  and  1,525  m.)  there  is  a  rapid  elimination  of  all  but  a  very 
few  of  the  characteristic  desert  species,  and  on  north  slopes  at  the 
latter  elevation  nearly  all  of  the  dominant  Encinal  forms  have  made 
their  entry.  The  upper  limit  of  the  Desert  may  be  placed  at  4,000  feet 
for  north  slopes  and  4,500  feet  (1,472  m.)  for  south  slopes.  The  upper 
edge  of  the  Desert  exhibits  an  attenuated  occurrence  of  all  of  the  larger 
desert  plants  and  the  presence  of  many  perennial  grasses  and  semi- 
woody  plants  which  occur  both  in  the  Encinal  Region  and  on  the  bajadas 
of  equal  or  slightly  greater  elevation  in  the  neighboring  portions  of 
Arizona.  The  extreme  upper  limit  of  desert  forms  is  7,000  feet  (2,133 
m.),  an  elevation  which  is  reached  by  a  single  succulent  species.  Follow- 
ing the  dissimilarity  of  the  lower  and  upper  portions  of  the  Desert  Region 
they  have  been  described  separately. 

The  Encinal  Region  extends  from  the  occurrence  of  the  first  ex- 
tremely open  groves  of  evergreen  oaks  on  north  slopes  at  4,000  feet  up 
to  the  first  elevation  at  which  the  larger  pines  begin  to  dominate  the 
physiognomy  of  the  vegetation,  at  about  6,300  feet  (1,920  m.)  on  south 


14  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

slopes.  A  few  of  the  dominant  species  of  the  Encinal  are  found  on 
the  higher  bajadas,  above  3,500  feet  (1,067  m.)  elevation,  but  in  the 
mountains  none  of  its  species  is  to  be  found  so  low  as  this  except  on 
north  slopes  or  near  arroyos  of  large  drainage  area.  At  4,000  feet,  on 
north  slopes,  several  of  the  larger  Encinal  plants  are  encountered,  and 
at  4,300  feet  (1,310  m.),  on  north  slopes,  several  additional  dominant 
species  are  found.  Within  the  Encinal  Region  it  is  possible  to  recognize 
a  lower  and  an  upper  portion,  distinguished  chiefly  by  the  openness  of 
the  former  and  the  closed  character  of  the  latter.  The  closed  Upper 
Encinal  merges  gradually  into  the  Forest,  losing  some  of  its  character- 
istic species  at  6,000  to  6,300  feet  (1,830  to  1,920  m.),  while  others  range 
to  7,800  feet  (2,380  m.)  and  a  few  to  8,300  feet  (2,530  m.)  on  south 
slopes. 

The  lowest  occurrence  of  Forest  is  on  north  slopes  at  about  5,800 
feet  (1,768  m.)  and  on  south  slopes  at  about  6,300  feet  (1,920  m.). 
The  Forest  is  at  first  rather  open  and  is  superposed,  as  it  were,  upon 
the  closed  Encinal,  but  it  becomes  heavier  and  the  Encinal  elements 
within  it  become  more  sparse  at  elevations  of  from  6,300  to  6,800  feet 
(1,920  to  2,073  m.),  according  to  the  slope  exposure.  The  upper  limit 
of  Forest  is  not  reached  in  the  Santa  Catalina  Mountains  at  their 
highest  elevation  of  9,150  feet  (2,790  m.),  nor  in  the  adjacent  Pinaleno 
Mountains  (Mount  Graham)  at  10,516  feet  (3,205  m.).  The  forest  of 
yellow  pine  occupies  all  south  slopes  up  to  the  summit  of  Mount 
Lemmon.  A  forest  dominated  by  fir,  spruce,  and  Mexican  white  pine 
occupies  the  north  slopes  above  7,500  feet  (2,287  m.),  the  earliest 
occurrence  of  these  species  being  about  1,000  feet  (305  m.)  lower. 

The  description  of  vegetation  which  is  given  in  the  following  pages 
applies  only  to  the  south  face  of  the  Santa  Catalinas.  The  north  face 
presents  more  abrupt  slopes  than  the  south,  with  most  of  its  ridges 
running  north  from  the  main  ridge.  This  circumstance  obscures  the 
influence  of  slope  exposure,  since  it  presents  opposed  slopes,  facing 
east  and  west,  which  are  identical  in  their  vegetation.*  Furthermore 
the  north  face  of  the  range  is  mineralogically  diversified,  presenting 
exposures  of  shale,  sandstone,  limestone,  diorite,  and  gneiss,  whereas 
the  south  face  presents  an  exposure  of  gneiss  only,  with  a  resultant 
mineral  identity  of  soils  from  base  to  summit.  It  has  thus  been  possible 
to  carry  out  a  study  of  climatic  influences  over  a  vertical  gradient  of 
6,000  feet  (1,830  m.)  with  uniform  soil,  and  the  east  and  west  ridges 
of  the  south  face  have  furnished  opposed  north  and  south  slopes  at  all 
elevations. 

*  Differences  between  the  vegetation  of  east  and  west  slopes  have  been  pointed  out  by  Blumer 
for  El  Rincon  Mountain,  but  the  differences  noted  were  of  another  character  from  those  commonly 
existent  between  north  and  south  slopes.  See:  Blumer,  J.  C.  A  Comparison  between  two  Moun- 
tain Sides.    The  Plant  World,  13:    134-140.    1910. 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  15 

THE  DESERT  REGION.* 

Under  the  designation  of  ''Desert"  are  comprised  all  those  portions 
of  the  Santa  Catalina  Mountains  in  which  the  vegetation  is  open,  low, 
and  diversified  in  the  assemblage  of  growth-forms,  with  a  predominance 
of  microphyllous  trees  and  shrubs  and  an  abundance  of  cacti.  Such 
a  vegetation  is  to  be  found  covering  the  upper  bajadas  and  extending 
up  the  slopes  of  the  mountain  to  elevations  of  4,000  to  4,500  feet 
(1,220  to  1,372  m.),  according  to  slope  exposure.  The  vegetation  of  the 
Upper  Bajadas  will  be  described  for  the  sake  of  the  contrast  which  it 
affords  with  the  vegetation  of  the  upper  portions  of  the  mountain,  as 
well  as  to  give  a  picture  of  the  plant  Ufe  by  which  the  mountain  is 
surrounded  and  from  which  it  has  derived  many  of  its  characteristic 
species.  The  desert  slopes  of  the  mountain  itself  exhibit  at  first  a  close 
resemblance  to  the  bajada,  and  then  lose  most  of  the  larger  bajada 
plants  before  the  entry  of  the  dominant  plants  of  the  Encinal  region. 
This  circumstance  admits  of  a  subdivision  of  the  Desert  region  of  the 
mountain  into  Lower  Desert  Slopes  and  Upper  Desert  Slopes.  The 
latter  region  is  much  poorer  than  the  former  in  cacti  and  much  richer 
in  grasses,  both  from  the  standpoints  of  the  number  of  species  and  the 
number  of  individuals.  The  Upper  Desert  is  similar  in  vegetation  to 
many  of  the  Upper  Bajadas,  such  as  those  to  the  northwest  of  the  Santa 
Catalinas  and  to  the  east  and  west  of  the  Santa  Rita  Mountains,  and 
might  well  be  designated  as  ''semi-desert"  or  "desert-grassland  transi- 
tion." It  is,  however,  essentially  similar  to  the  desert  plains  in  its 
vegetational  make-up,  and  in  no  part  of  Arizona  does  it  serve  as  a 
transition  to  true  grassland.  The  largest  canons  of  the  Santa  CataUnas 
possess  some  plant  communities  that  are  radically  unlike  the  vegeta- 
tion of  the  desert  itself,  but  not  unlike  the  communities  which  surround 
the  springs  and  wells  of  the  desert  plains.  These  are  the  groups  of 
aquatic  and  palustrine  plants  which  accompany  the  streams  and  pools 
of  the  cafions.  The  smaller  caiions  and  arroyos  t  present  distinctive 
features  of  vegetation,  departing  more  and  more  from  the  large  canons 
and  approaching  more  nearly  the  character  of  the  desert  areas  away 
from  water.  All  of  these  areas  have  been  treated  as  a  part  of  the  Desert 
Region. 

THE  UPPER  BAJADAS. 

The  Lower  Bajadas  of  the  Tucson  region  are  covered  by  a  vegeta- 
tion in  which  Covillea  tridentata  (jediondia,  creosote  bush)  is  always  the 
predominant  plant  and  is  often  almost  the  sole  plant  of  more  than 
2  feet  in  height  over  areas  many  square  miles  in  extent.  The  plants 
which  most  commonly  enter  this  community  are  Prosopis  velutina 
(mesquite),  Opuntia  fulgida,  Opuntia  spinosior  (both  arborescent  cyiin- 

*  The  word  "region"  is  not  here  used  in  any  of  the  technical  senses  in  which  it  has  been  em- 
ployed in  phytogeography. 

t  The  Spanish  word  arroyo  is  in  common  use  in  the  southwestern  United  States  to  designate 
Btreamways  which  are  usually  without  water. 


16  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

dropuntias),  Opuntia  toumeyi,  Opuntia  blakeana  (procumbent  plato- 
pimtias),  and  Acacia  paucispina. 

The  shorter  and  steeper  Upper  Bajadas  which  fringe  the  southern 
and  southwestern  edge  of  the  Santa  CataUnas  are  clothed  with  a  much 
more  diversified  vegetation,  in  all  respects  similar  to  that  of  other 
Upper  Bajadas  which  lie  below  3,500  feet  (1,067  m.)  in  other  localities 
in  southwestern  Arizona.  The  freedom  of  the  soil  from  caliche  is  here, 
as  elsewhere,  responsible  for  the  existence  of  a  diversified  vegetation 
rather  than  a  pure  stand  of  Covillea. 

The  Upper  Bajadas,  as  exemplified  along  the  south  face  of  the  Santa 
Catalinas  at  about  3,000  feet  elevation  (915  m.),  bear  what  may  be 
regarded  in  many  respects  as  the  most  highly  developed  type  of  desert 
vegetation  to  be  found  in  southern  Arizona  or  northern  Sonora.  In  the 
Upper  Bajadas  may  be  found  a  greater  number  of  species  of  perennial 
plants  than  in  any  other  distinctly  desert  situations.  In  them  also 
the  number  of  individual  perennial  plants  per  unit  area  is  greater  than 
in  any  areas  outside  the  flood  plains  of  such  rivers  as  the  Santa  Cruz 
and  Gila.  The  only  areas  that  compare  with  the  High  Bajadas  in 
these  respects  are  the  volcanic  hills  in  which  basaltic  rock  has  weathered 
to  a  fine  clay  which  is  very  retentive  of  soil  moisture,  as  is  well  exempli- 
fied in  Tumamoc  Hill,  the  site  of  the  Desert  Laboratory.  The  andesitic 
and  rhyolitic  hills  in  the  vicinity  of  Tumamoc  are  much  poorer  than 
it  is  in  the  number  of  individual  plants  per  unit  area,  although  perhaps 
nearly  as  rich  in  their  flora. 

On  the  Upper  Bajadas  there  often  occur,  in  almost  equal  admixture, 
from  15  to  25  perennial  species  of  plants  of  such  size  as  to  dominate  the 
physiognomy  of  the  vegetation.  These  same  species  may  be  found  on 
the  more  nearly  level  Lower  Bajadas,  but  any  one  of  them  may  often 
be  absent  for  many  miles,  may  be  sporadically  represented  by  a  few 
individuals,  or  may  occur  in  dense  but  local  colonies  (particularly  in 
the  case  of  the  cacti).  Occasionally  as  many  as  5  to  10  of  the  species 
may  be  within  sight  at  the  same  time. 

The  flora  which  characterizes  the  Upper  Bajadas  of  the  Santa 
Catalinas  ranges  without  substantial  loss  down  to  sea-level  on  the  gulf 
of  Cahfornia,*  and  the  vegetation  formed  by  their  commingling  may 
be  found  as  a  belt  covering  the  high  bajadas  which  encircle  all  of  the 
mountain  ranges  and  clothing  all  of  the  low  basaltic  hiUs.  A  climb  of 
2  hours  from  the  base  of  the  Santa  Catalinas  will  discover  greater 
changes  of  vegetation  and  flora  than  can  be  encountered  in  the  150 
miles  (242  km.)  between  Tucson  and  Adair  Bay. 

The  Upper  Bajadas  present  the  desert  characteristics  of  openness 
of  stand,  lowness  of  stature,  and  commingling  of  diverse  vegetation 
types.    The  first  of  these  features  is  common  to  the  vegetation  of  all 

*SeeHornaday,W.T.  Camp  Fires  on  Desert  and  Lava.  New  York,  Scribner,  1909.  MaoDougal, 
D.  T.    Across  Papagueria.    The  Plant  World,  11:  93-99,  123-131,  1908. 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  17 

deserts,  the  last  is  at  least  characteristic  of  the  less  pronounced  deserts 
of  the  southwestern  United  States  and  of  Mexico.  The  openness  of  the 
stand  is  such  that  it  is  possible  in  all  places  to  ride  a  horse  through  the 
vegetation  and  to  take  whatever  course  the  rider  may  wish,  with  only- 
occasional  digressions  of  a  few  yards  from  the  general  direction  of 
travel.  The  stature  of  the  vegetation  is  such  that  it  would  be  possible 
for  the  rider  to  keep  almost  constantly  in  view  another  mounted  man 
half  a  mile  distant.  The  columnar  giant  cacti  reach  a  maximum  height 
of  40  feet  (12  m.)  and  the  trees  a  height  of  20  to  25  feet  (6  to  8  m.). 
The  great  bulk  of  the  shrubs  and  succulents,  however,  are  not  more 
than  6  feet  (2  m.)  in  height,  and  many  of  them  are  less  than  4  feet 
(1.2  m.) .  Among  the  commonest  vegetation  types  are  stem-succulents, 
microphyllous  and  sclerophyllous  trees  and  shrubs,  macrophyllous  decid- 
uous shrubs,  perennial  grasses,  and  root-perennial  and  ephemeral  her- 
baceous plants. 

Largest  and  most  conspicuous  of  the  succulents  is  Carnegiea  gigantea 
(saughuaro,  giant  cactus),  which  is  here  in  its  optimum  habitat  and 
very  abundant  (plate  3,  b)  .  Among  the  microphyllous  trees  the  most 
abundant  are  Prosopis  velutina,  Acacia  greggii,  Acacia  paucispina,  and 
the  green-barked  Parkinsonia  microphylla  (palo  verde).  The  much- 
branched  arborescent  types  of  cacti  are  represented  by  Opuntia  versi- 
color, which  attains  a  maximum  height  of  12  feet  (4  m.),  and  by  Opuntia 
fulgida  and  Opuntia  mamillata  (cholla),  remarkable  for  the  brilliance 
of  their  glistening  straw-colored  spines.  Opuntia  blakeana,  Opuntia 
engelma7ini,  Opuntia  toumeyi,  and  Opuntia  discata  are  abundant  repre- 
sentatives of  the  platopuntia  group.  The  evergreen  Covillea  is  greatly 
outnumbered  by  Fouquieria  splendens  (ocotillo).  The  globular  Echino- 
cactus  wislizeni  (bisnaga)  attains  a  height  of  4  feet  (1.3  m.)  with  an 
even  greater  girth.  Similar  in  form  but  never  exceeding  a  foot  in 
height  are  Echinocereus  fendleri  and  Mamillaria  grahami.  The  sclero- 
phyllous Simmondsia  calif  arnica  (jojobe)  and  the  relatively  large-leaved 
deciduous  Jatropha  cardiophylla  are  frequent,  while  a  large  number  of 
less  striking  shrubs  are  common,  including  Franseria  deltoidea,  Isocoma 
hartwegi,  Encelia  farinosa,  Zizyphus  lycioides  var.  canescens,  Lycium 
torreyi,  Momisia  pallida,  Krameria  glandulosa,  Trixis  angustifolia  var. 
latiuscula,  Crassina  pumila,  and  Psilostrophe  cooperi. 

The  seasonal  rains  of  winter  and  those  of  summer  cause  activity  of 
foliation  and  growth  on  the  part  of  all  of  the  smaller  shrubs.  The 
winter  rains  cause  foliation  in  Parkinsonia  and  Fouquieria,  but  not  in 
Prosopis  and  the  species  of  Acacia.  Neither  do  they  initiate  growth 
in  Parkinsonia,  Fouquieria,  nor  any  of  the  cacti.  The  two  widely 
separated  seasons  of  rain  bring  forth  two  wholly  distinct  sets  of  herba- 
ceous ephemeral  plants,  at  the  same  time  that  each  season  causes  activ- 
ity upon  the  part  of  some  of  the  root-perennials.  The  ephemeral  plants 
may  form  a  dense  carpet  over  both  the  Upper  Bajadas  and  the  Lower 


18 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


Bajadas  in  seasons  of  well-distributed  and  copious  rainfall.  With  less 
favorable  conditions  they  may  form  a  very  light  cover  or  may  be  almost 
absent.  The  total  flora  of  root-perennials  and  ephemerals  is  large,  and 
the  relative  abundance  of  the  various  species  fluctuates  tremendously 
from  spot  to  spot,  and  in  the  same  spot  it  is  by  no  means  the  same  from 
year  to  year.  This  flora  is  nearly  identical  with  that  of  the  basaltic  hills 
in  the  vicinity  of  Tucson,  and  has  been  fully  listed  by  Thornber,*  with 
a  subdivision  of  vegetation  types. 

In  the  following  list  have  been  brought  together  the  names  of  the 
most  characteristic  plants  of  the  Upper  Bajadas,  grouped  vegetistically 
and  briefly  described.  Asterisks  indicate  the  relative  abundance  of 
the  species — three  indicating  that  a  plant  is  extremely  common,  two 
that  it  is  very  common,  and  one  that  it  is  fairly  common.  Figures 
follow  the  descriptions,  indicating  the  average  height  of  each  species. 
A  comparison  of  all  the  maximum  heights  given  will  convey  an  impres- 
sion of  the  low  stature  of  the  commonest  components  of  the  vegetation. 


Vegelistic  Grouping  of  the  Characteristic  Species  of  the  Upper  Bajadas. 


Perennial  Non-succulent  Trees  and  Shrubs: 
***  Acacia  greggii,  microphyllous,  winter- 
deciduous.    2  to  3  m. 
**  Covillea     tridentata,      microphyllous, 
evergreen.    1  to  2.5  m. 

*  Crossosoma   bigelovii,    sclerophyllous, 

evergreen.    1  to  1.5  m. 

*  Ephedra   trifurca,    aphyllous,    green- 

stemmed.     0.5  to  1  m. 
***  Fouquieria  splendens,  macrophyllous, 

drought-deciduous.    2  to  4  m. 

**  Jatropha  cardiophylla,  macrophyllous, 

winter-deciduous.     0.5  to  1  m. 

*  Kceherlinia  spinosa,  aphyllous,  green- 

stemmed.     0.5  to  1  m. 
**  Krameria  glandulosa,   microphyllous, 

evergreen.    1  to  2  m. 
**  Lycium    berlandieri,     microphyllous, 

evergreen.    1  to  2  m. 

*  Lycium     fremontii,      microphyllous, 

evergreen.    1  to  2  m. 
**  Momisia  pallida,  sclerophyllous,  ever- 
green.   1.5  to  2.5  m. 

*  Olneya   tesota,    microphyllous,    ever- 

green (foliage  occasionally  winter- 
killed).   3  to  6  m. 

***  Parkinsonia  microphylla,  microphyl- 
lous, drought-deciduous,  green- 
stemmed.    2  to  5  m. 

***  Prosopis      velutina,      microphyllous, 
winter-deciduous.    3  to  6  m. 
**  Zizyphus     lycioides     var.    canescens, 
microphyllous,  evergreen  1  to  2  m. 


Perennial  Succulent  Plants: 

***  Carnegiea        gigantea,         columnar 

branched.    5  to  14  m. 
**  Echinocactus     wislizeni,     cylindrical 

0.5  to  1.5  m. 
**  Echinocereus  fendleri,  cylindrical,  cse 

spitose.    0.1  to  0.4  m. 
**  Mamillaria  grahami,  cylindrical,  soli 

tary  or  csespitose.    0.1  to  0.2  m. 
**  Opuntia  blakeana,   flat-jointed,   prO' 

cumbent. 
***  Opuntia    discata,     flat-jointed,     pro 

cumbent  or  semi-erect. 
**  Opuntia    fulgida,    cylindrical-jointed 

arborescent,  1  to  2  m. 
***  Opuntia       mamillata,        cyUndrical 

jointed,  arborescent.    1  to  2  m. 
**  Opuntia    toumeyi,    flat-jointed,    pro 

cumbent. 
***  Opuntia  versicolor,  cylindrical-jointed 
arborescent.    1  to  4  m. 
Perennial  Shrublets  (all  less  than  0.7  m.  high) 

*  Coldenia  canescens,  sclerophyllous. 
**  Crassina  pumila,  dissected  leaves. 

***  Encelia      farinosa,      macrophyllous, 

slightly  drought-deciduous. 
***  Franseria  deltoidea,  sclerophyllous. 
***  Isocoma  hartwegi,  dissected  leaves. 
***  Kalliandra  eriophylla,  dissected  leaves 
**  Psilostrophe  cooperi,  sclerophyllous. 

*  Trixis    angustifolia    var.     laiiuscula 

sclerophyllous,    shghtly    drought- 
deciduous. 


*  Thornber,  J.  J.    Vegetation  Groups  of  the  Desert  Laboratory  Domain. 
Wash.  Pub.  113,  Chapter  IV,  1909. 


Carnegie    Inst. 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  19 

Ephemeral  Summer-active  Herbaceous  Plants — Continued. 


Root  Perennials  {all  facultative  evergreens): 
***  Abutilon  incanum,  sclerophyllous. 
**  Brodioea     capitata     var.     pauciflora, 

bulbous,  linear  leaves. 
**  Cassia  coyesu,sclerophylIous,branched 
leaves. 

*  Dalea  parryi,  microphyllous. 

***  Muhlenbergia  porteri,  semi-scandent. 
**  Pentstemon  wrightii,  macrophyllous. 
**  Perezia  wrightii,  macrophyllous. 
**  Verbena  ciliata,  macrophyllous,  hairy. 
Ephemeral  Summer-active  Herbaceous  Plants: 
**  Bahia  absinthifolia. 
***  Baileya  multiradiata. 
**  Boerhaavia  pterocarpa. 

*  Boerhaavia  walsoni. 
***  Bouteloua  arislidoides. 
***  Cladothrix  lanuginosa. 

**  Euphorbia  fiorida. 

*  Euphorbia  melanadenia. 


Ephemeral  Summer-active  Herbaceaus  Plants: 
Continued. 
***  Pedis  papposa. 
***  Wedelia  incarnata. 
Ephemeral  Winter-active  Herbaceous  Plants. 
**  Actinolepis  lanosa. 
**  Anisololus  trispermus. 
***  Baeria  chrysostoma. 
**  Chorizanthe  brevicornu. 
**  Cryptanthe  intermedia. 

*  Eremocarya  micrantha. 
***  Gilia  floccosa. 

**  Lepidium  lasiocarpum. 
**  Lesquerella  gordoni. 

*  Mentzelia  albicaulis. 

*  Orthocarpus  purpurascens. 
**  Pectocarya  linearis. 

***  Plantago  fastigiata. 
***  Plantago  ignota. 


THE  DESERT  ARROYOS  AND  CANONS. 

In  crossing  the  Upper  Bajadas  it  is  often  possible  to  detect,  by  means 
of  the  vegetation,  the  approach  to  a  very  shallow  drainageway  through 
which  water  runs  for  only  a  few  hours  after  the  severest  summer  rains. 
The  larger  arroyos  are  still  more  conspicuous  by  reason  of  the  still 
heavier  stand  of  vegetation  along  their  margins,  and  in  the  largest 
caiions  is  found  the  culmination  of  the  influence  of  surface  streams 
and  underflows  for  the  support  of  vegetation.  The  effect  of  the  most 
transitory  of  the  small  streams  is  merely  the  raising  of  the  moisture  of 
adjacent  soil  to  such  a  point  that  it  will  present  favorable  conditions 
for  plant  activity  for  a  longer  time  after  the  close  of  the  rainy  periods 
than  will  the  soil  of  the  bajada  in  general.  There  is  only  a  negligible 
and  short-lived  underflow  in  these  smallest  arroyos,  and  their  only 
differences  from  the  bajada  are  that  in  the  rainy  seasons  they  present 
sHghtly  more  favorable  conditions  with  respect  to  soil  moisture  and 
that  the  effect  of  the  rainy  season  is  slightly  prolonged  in  them,  while 
the  periods  of  drought  are  correspondingly  shortened.  In  the  larger 
arroyos  there  may  not  be  a  constant  underflow,  but  there  is  at  least  a 
relatively  high  percentage  of  soil  moisture  for  periods  of  sufficient  length 
to  greatly  reduce  the  influence  of  the  arid  periods  upon  their  plants. 
In  the  largest  arroyos  and  in  the  mountain  canons  themselves  there  is 
either  a  constant  underflow,  maintaining  high  moistures  in  the  soil  of 
the  banks  and  bed  of  the  arroyo,  or  else  there  is  constant  water,  either 
running  or  standing  in  pools. 

The  smallest  arroyos,  which  are  very  frequent  on  the  Upper  Bajada 
in  the  close  proximity  of  the  mountain,  present  no  peculiar  species,  but 
merely  a  closer  stand  of  the  same  plants  that  are  to  be  observed 
throughout  the  bajada,  notably  Prosopis,  Acacia  greggii,  and  Momisia 
pallida.  Along  somewhat  larger  arroyos  are  to  be  found  still  heavier 
stands  of  the  above  species,  together  with  Parkinsonia  torreyana,  Celtis 


20  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

reticulata,  Baccharis  sarothroides  (batamote),  Franseria  ambrosioides, 
Lycium  fremontii,  Verhesina  encelioides,  and  Behhia  juncea. 

In  the  canons  and  arroyos  large  enough  to  have  a  heavy  flow  of 
storm  water  but  not  large  enough  to  have  even  pools  of  water  which 
are  constant  throughout  the  year,  there  may  be  found  several  additional 
species  of  plants  which  also  occur  on  the  sandy  flood-plains  of  the 
largest  canons.  Prominent  among  these  are  Chilopsis  linearis,  Hymeno- 
clea  monogyra,  and  Baccharis  glutinosa,  all  of  which  are  large  shrubs 
or  in  the  case  of  Chilopsis  may  attain  the  size  of  small  trees.  Also 
characteristic  of  these  sands  are  Franseria  ambrosioides,  Rumex  hymeno- 
sepalus,  Euphorbia  pediculifera.  Clematis  ligusticifolia,  and  Calyptridium 
monandrum. 

In  the  largest  canons  of  the  south  side  of  the  Santa  Catalinas  it  is 
possible  to  witness  the  occurrence  of  communities  of  mesophilous, 
palustrine,  and  aquatic  plants  which  are  limited  in  area  but  are  made 
up  of  species  which  stand  strongly  in  contrast  with  the  predominant 
forms  of  the  bajadas.  The  existence  of  streams  and  pools  adjacent  to 
rocky  slopes  makes  it  possible  in  several  places  for  Callitriche  and 
Isnardia  to  grow  within  20  feet  of  Carnegiea  and  Fouquieria. 

At  the  mouth  of  Soldier  Canon  the  rocky  slopes  of  the  streamway 
are  clothed  with  typical  bajada  plants  together  with  a  few  forms  which 
are  particularly  abundant  on  cliffs  and  in  rocky  situations,  both  in  the 
larger  mountains  of  the  region  and  in  the  volcanic  hills.  Among  the 
latter  are  Opuniia  bigelovii,  Hyptis  emoryi,  Lippia  wrightii,  Anisacan- 
thus  thurberi,  Encelia  farinosa,  Eriogonum  wrightii,  Chrysoma  laricifolia, 
and  Crossosoma  bigelovii.  Among  the  boulders  bordering  the  stream- 
way  are  Janusia  gracilis,  Plumbago  scandens,  Maurandia  antirrhini- 
flora,  Mimitanthe  pilosa,  and  Stemodia  plumieri,  as  well  as  occasional 
individuals  of  several  species  which  are  common  away  from  streams 
at  elevations  of  4,000  to  5,000  feet,  as,  for  example,  Dasylirion  wheeleri, 
Nolina  microcarpa,  Erythrina  flabelliformis,  Ingenhousia  triloba,  and 
Mimosa  biuncifera.  When  the  sands  of  the  arroyo  have  not  been 
recently  scoured  by  floods  they  support  scattered  individuals  of  Ama- 
ranthus  palmeri  (celite).  Cassia  leptocarpa,  Nicotiana  trigonophylla, 
Bebbia  juncea,  Hymenoclea  monogyra,  Franseria  xanthocarpa,  Asclepias 
linifolia,  Baccharis  sarothroides,  Mentzelia  gracilenta,  and  Carduus  sp. 

In  Ventana,  Bear,  and  Sabino  Canons  it  is  possible  at  all  times  of 
the  year  to  find  small  colonies  of  palustrine  and  aquatic  plants,  and 
the  vicinity  of  such  localities  is  the  optimum  habitat  for  Prosopis  and 
Populus.  An  underflow  passes  out  at  the  mouth  of  Sabino  Caiion  which 
is  heavier  and  more  constant  than  that  of  any  other  canon  in  the  range; 
this  gives  Sabino  Canon  its  abundant  mesophilous  vegetation  and  also 
causes  the  arroyo  through  which  its  flood  waters  reach  the  Rillito  to 
be  occupied  by  a  much  richer  stand  of  vegetation  than  is  to  be  found 
along  any  of  the  arroyos  of  the  adjacent  region.     The  sandy  and 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  21 

boulder-strewn  bed  is  from  100  to  300  feet  in  width  and  the  portions 
covered  by  the  storm  floods  vary  from  season  to  season.  Trees  not 
only  occupy  the  banks  of  the  arroyo  but  occur  scattered  throughout 
its  bed,  where  they  may  persist  for  many  years,  only  to  be  eventually 
uprooted  by  some  exceptionally  severe  freshet.  Among  these  trees 
are  Populus  sp.,  Fraxinus  tourney i,  Juglans  major,  Platanus  wrightii, 
Sapindus  drummondii,  Prosopis  velutina,  and  Samhucus  mexicana,  some 
of  which  reach  a  height  of  50  to  60  feet  (15  to  18  m.).  Such  shrubs  as 
Chilopsis  linearis,  Baccharis  sarothroides,  Baccharis  glutinosa,  and 
Hymenoclea  monogyra  are  also  common  in  and  along  this  arroyo. 

The  floor  of  Sabino  Canon  from  its  mouth  to  the  Sabino  Basin  is 
occupied  by  an  irregular  and  broken  forest  of  Prosopis,  Populus,  Frax- 
inus, Platanus,  and  Salix  {Salix  wrightii  and  Salix  sp.).  Among  these 
common  trees  are  scattered  a  few  individuals  of  three  of  the  oaks 
characteristic  of  the  Encinal  region:  Quercus  ohlongifolia,  Quercus 
arizonica,  and  Quercus  emoryi.  These  oaks  occur  near  the  mouth  of 
the  canon  at  an  elevation  of  2,700  feet  (823  m.),  although  their  lowest 
occurrence  on  slopes,  away  from  the  proximity  of  an  underflow,  is  at 
4,200  to  4,500  feet  (1,280  to  1,372  m.).  Cupressus  arizonica  occurs  in 
the  upper  half  of  the  canon  and  in  Sabino  Basin  at  elevations  above 
3,200  feet  (975  m.).  It  is  confined  to  the  proximity  of  streams  up  to 
an  elevation  of  6,000  feet,  above  which  it  is  occasionally  found  on 
slopes. 

The  shrubby  vegetation  of  the  floor  of  Sabino  Canon  includes  all 
of  the  species  which  have  been  mentioned  as  occurring  in  the  smaller 
canons  and  arroyos,  together  with  a  number  of  shrubs  and  perennials 
which  are  more  common  along  the  arroyos  and  streams  of  the  Encinal 
region.  Among  the  latter  are  those  species  mentioned  as  occurring 
in  Soldier  Canon  and  also  Viiis  arizonica,  Bouvardia  triphylla,  Amorpha 
calif ornica,  and  Brickellia  calif ornica. 

Other  characteristic  plants  are  the  shrubs  Dodoncea  viscosa  var. 
angustifolia,  Eysenhardtia  orthocarpa,  Indigofera  sphcerocarpa,  and  the 
woody  climber  Nissolia  schottii. 

Among  the  herbaceous  plants  common  in  and  along  the  pools  and 
water-courses  of  Sabino  Canon  may  be  mentioned : 


CalUtriche  8p. 
Carex  sp. 

Cerastium  texanum. 
Cyperus  inflexus. 
Helenium  thurberi. 
Hydrocotyle  ranunculoides. 
Isnardia  palustris. 
Juncus  arizonicus. 
Juncus  bufonius. 
Juncus  interior. 
Juncus  sphcerocarpus. 
Linaria  canadensis. 
Mecardonia  peduncularis. 


Mimulus  langsdorfii. 
Mimitanthe  pilosa. 
Montia  perfoliata. 
Myosurus  cupulalus. 
Phalaris  intermedia. 
Platystemon  californicus. 
Polygonum  sp. 
Senecio  lemmoni. 
Specularia  biflora. 
Stachys  coccinea. 
Stemodia  durantifolia. 
Tagetes  lemmoni. 
Tilloea  erecta. 


22  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

THE  LOWER  DESERT  SLOPES. 

On  leaving  the  uppermost  edge  of  the  bajada  and  commencing  the 
ascent  of  the  mountain  over  the  rather  abrupt  slopes  which  lie  between 
the  larger  canons,  a  region  is  entered  upon  in  which  the  physical  con- 
ditions differ  from  those  of  the  bajada  chiefly  in  the  pronounced  slope 
exposure  to  the  south,  southwest,  or  southeast,  and  in  the  occurrence 
of  large  masses  of  rock  in  situ,  with  the  coarse  soil  limited  to  small 
benches,  pockets,  and  fissures.  The  vegetation  of  these  lowest  slopes 
is  very  similar  to  that  of  the  Upper  Bajadas,  and  is  composed  of  a 
nearly  identical  flora.  Prosopis,  Parkinsonia,  and  Acacia  are  repre- 
sented by  smaller  and  less  frequent  individuals,  and  both  the  cylindro- 
puntias  and  platopuntias  occur  somewhat  less  frequently.  Carnegiea 
gigantea  is  even  more  abundant  on  the  slopes  than  on  the  bajadas, 
being  represented  by  smaller  individuals,  among  which  relatively  few 
have  reached  the  size  at  which  branching  begins.  For  Carnegiea  and 
the  above-mentioned  trees  the  relatively  rapid  erosion  of  the  soft  gneiss 
and  the  shifting  of  the  shallow  soil  are  apparently  too  great  to  permit 
the  attainment  of  great  size  or  age.  Fouquieria,  Encelia,  and  Chrysoma 
laricifolia  are  even  more  abundant  on  the  slopes  than  on  the  Upper 
Bajada,  and  Opuntia  bigelovii,  the  most  densely  spiny  of  all  the  cylin- 
dropuntias,  is  found  exclusively  on  southerly  slopes  and  cliffs,  in  very 
rocky  substratum,  at  elevations  below  3,500  feet  (1,067  m.).  Olneya 
tesota  and  Covillea  have  not  been  detected  on  the  mountain  slopes  (see 
plates  4  and  5). 

The  summer  and  winter  ephemerals  of  the  bajada  are  nearly  all 
to  be  found  on  the  Desert  Slopes  of  the  mountain,  but  rarely  in  such 
abundance  as  they  attain  on  level  ground.  Among  the  most  common 
of  the  ephemerals  and  root-perennials  to  be  observed  in  the  summer 
are  Cladothrox  lanuginosa,  Pedis  papposa.  Euphorbia  florida,  Bcer- 
haavia  pterocarpa,  Bouteloua  aristidoides,  Andropogon  sacchar aides, 
Wedelia  incarnata,  Machoer  anther  a  tanacetifolia,  Triodia  mutica,  Evol- 
vulus  arizonicus,  Allionia  gracillima,  and  Cassia  covesii.  The  bases  of 
boulders  and  partially  shaded  ledges  of  rock  are  the  habitats  of  Selagi- 
nella  rupincola,  Cheilanthes  lindheimeri,  and  Notholcena  hookeri.  The 
ferns  are  not  common  and  are  conspicuous  only  during  rainy  periods, 
but  the  Selaginella  is  abundant  here  and  becomes  even  more  so  at 
slightly  higher  elevations,  where  it  frequently  clothes  the  rocky  walls 
of  steep  canons  to  such  an  extent  that  their  usual  grayness  is  converted 
to  a  vivid  green  a  few  hours  after  a  heavy  rain  (see  plate  7) . 

The  ascent  from  3,500  to  4,000  feet  (1,067  to  1,220  m.)  witnesses 
the  first  essential  changes  in  the  vegetation.  At  the  latter  elevation 
nearly  all  of  the  typical  desert  forms  may  be  found,  but  Opuntia  has 
become  infrequent  and  Carnegiea  gigantea,  Echinocactus  wislizeni,  and 
Fouquieria  splendens  are  conspicuously  confined  to  southerly  slopes 
(see  plates  6  and  8) .    Parkinsonia  torreyana,  which  is  confined  to  arro- 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  23 

yos  on  the  bajada,  is  found  here  growing  with  Parkinsonia  microphylla, 
which  it  eventually  exceeds  in  vertical  distribution  by  nearly  500  feet 
(153  m.).  Prosopis  is  even  more  abundant  at  4,000  feet  (1,220  m.) 
than  it  is  on  the  lowest  slopes,  and  attains  a  trunk  circumference  of 
6  feet  (2  m.)  at  4,200  feet  elevation  (1,280  m.),  within  600  vertical 
feet  (183  m.)  of  its  upper  limit.  Such  common  shrubs  of  the  bajada 
as  Lycium,  Zizyphus,  Krameria,  Jatropha,  and  Momisia  are  now  very 
sporadic  in  their  occurrence,  and  the  compact,  hemispherical  and 
vividly  green  Chrysoma  laricifolia  has  become  very  frequent  and  con- 
spicuous, together  with  the  white-tomentose  Artemisia  ludoviciana  and 
the  less  conspicuous  Eriogonum  wrightii. 

On  northerly  slopes,  just  below  4,000  feet  (1,220  m.),  are  encountered 
the  first  individuals  of  the  rosaceous  tree  Vauquelinia  calif ornica  and 
of  Agave  palmeri  and  Dasylirion  wheeleri  (sotol).  Along  the  arroyos 
the  most  conspicuous  forms  are  Erythrina  flahelliformis,  the  large  leaves 
and  brilliant  scarlet  flowers  of  which  recall  its  tropical  congeners, 
Manihot  carthaginensis,  with  leaves  of  striking  form,  and  Ingenhousia 
triloba  (wild  cotton),  with  tripartite  leaves  and  large  white  flowers 
which  strikingly  resemble  those  of  the  cotton  plant. 

THE  UPPER  DESERT  SLOPES. 

The  slopes  lying  between  4,000  and  4,500  feet  (1,220  and  1,372  m.) 
constitute  the  upper  edge  of  the  desert.  On  these  slopes  all  the  char- 
acteristic species  of  the  bajada  are  confined  to  southerly  slopes,  and 
all  but  half  a  dozen  of  them  find  their  uppermost  limits.  On  the  Upper 
Desert  slopes  Vauquelinia  becomes  conomon,  although  confined  to 
ledges  of  rock,  and  Juniperus  pachyphlcea,  Quercus  oblongifolia,  and 
Quercus  arizonica  occur  for  the  first  time  away  from  canons.  On  the 
northerly  slopes,  where  these  trees  form  the  lowest  attenuated  edge  of 
the  Encinal  region,  Dasylirion  occurs  in  abundance  together  with  the 
lowest  individuals  of  Nolina  microcarpa  (bear  grass),  Arctostaphylos 
pungens  (manzanita).  Agave  schottii,  and  Yucca  macrocarpa. 

The  physiognomy  of  the  Upper  Desert  slopes  is  made  distinctive 
from  that  of  the  Lower  Desert  slopes  not  only  by  the  entrance  of  these 
plants  of  striking  form,  and  the  exit  of  the  desert  species,  but  also  by 
the  abundance  of  perennial  grasses,  root-perennials,  and  small  shrubs, 
which  combine  with  the  ephemeral  plants,  or  their  dead  remains,  to  give 
a  much  more  complete  ground  cover  than  is  to  be  found  in  any  part  of 
the  bajadas.  The  compact  and  extended  patches  oi  Agave  schottii  are  an 
important  element  in  this  low  cover,  and  so  are  the  scattered  plants  of 
Boutelouarothrockii  and  the  bunches  of  Boutelouacurtipendula,  Bouteloua 
oligostachya,  Muhlenbergia  dumosa,  Andropogon  scoparium,  Eragrostis 
lugens,  and  Heteropogon  contortus  (see  plates  8  and  9). 

Commonest  among  the  low  shrubs  and  other  perennials  of  the  Upper 
Desert  are:     Chrysoma  laricifolia,  Acacia  suffrutescens,   Eriogonum 


mPERU  UBi^^rV 


24  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

wrightii,  Dalea  wislizeni,  Calliandra  eriophylla,  Hymenopappus  mexi- 
canus,  Artemisia  ludoviciana,  Dalea  alhiflora,  Asclepias  linifolia,  Fran- 
seria  tenuifolia,  Baccharis  thesioides,  Ayenia  microphylla,  and  Anisolotus 
argensis.  The  commonest  summer  ephemerals  are  Eriogonum  abertia- 
num  and  Eriocarpum  gracile. 

The  flood-plains  and  the  banks  and  beds  of  the  arroyos  in  the  Upper 
Desert  are,  in  general,  more  like  the  arroyos  of  the  desert  in  their 
vegetation  than  like  those  of  the  Encinal  region.  The  largest  tribu- 
taries of  Sabino  Canon  are  somewhat  less  rich  in  aquatic  and  palustrine 
plants  than  the  lower  portion  of  the  canon,  and  merely  because  of 
their  steeper  gradient  and  less  regular  flow.  The  forest  which  occupies 
the  flood-plains  of  Sabino  Basin  is  chiefly  made  up  of  Quercus  emoryi, 
Quercus  arizonica,  Platanus  wrightii,  and  Cupressus  arizonica.  The 
smaller  flood-plains  and  arroyos  of  the  Upper  Desert  have  few  of  these 
trees  but  occasional  individuals  of  Populus  and  open  thickets  of  Bac- 
charis emoryi  and  Baccharis  sarothroides,  together  with  Franseria 
amhrosioides,  Ingenhousia  triloba,  Erythrina  fiabelliformis,  Croton  texen- 
sis,  Calliandra  eriophylla,  Brickellia  californica,  Gymnosperma  corym- 
bosa,  Amorpha  californica,  Bouvardia  triphylla,  and  Stachys  coccinea 
(see  plate  10a). 

THE  ENCINAL  REGION. 

Some  of  the  distinctive  species  of  the  Lower  Encinal  are  found  at  4,000 
feet  and  other  forms,  characteristic  of  the  Upper  Encinal,  extend  upward 
into  the  Forest  Region  as  far  as  8,000  to  8,600  feet.  The  Lower  Encinal 
may  be  said  to  have  its  commencement,  however,  in  the  open  orchard- 
like stands  of  Quercus  oblongifolia  and  Quercus  arizonica,  which  occupy 
northerly  slopes  at  about  4,300  feet.  At  an  approximate  elevation  of 
5,000  feet  the  open  Encinal  may  be  found  on  all  slopes  except  the  steepest 
southerly  ones,  while  on  steep  northern  slopes  it  already  forms  nearly 
closed  stands.  The  dense  stands  of  the  Upper  Encinal  region  begin  to 
appear  on  southerly  slopes  at  about  5,800  feet,  and  persist  to  the  eleva- 
tion of  6,200  to  6,400  feet,  where  large  pines  begin  to  dominate  the 
physiognomy  and  the  true  forest  may  be  said  to  begin. 

The  activities  of  growth  and  flowering  which  are  so  conspicuous  on 
the  Desert  in  the  season  of  winter  rains  are  very  much  reduced  in  the 
Lower  Encinal  and  are  practically  absent  in  the  Upper  Encinal. 
Leaves  are  retained  by  the  evergreen  oaks  and  the  sclerophyllous  shrubs 
throughout  the  winter  and  are  shed  in  April  or  May,  simultaneously 
with  the  first  growth  of  shoots  and  the  renewal  of  foliage.  Extremely 
few  of  the  ephemerals  which  often  carpet  the  Desert  in  January  are 
to  be  found  in  the  Encinal  region.  There  is  some  activity  on  the  part 
of  root  perennials  in  the  Lower  Encinal  during  the  months  of  March 
and  April,  and  flowers  may  be  found  on  species  of  Sphoeralcea,  Calo- 
chortus.  Verbena,  Pentstemon,  Eriogonum,  and  Lesquerella.  Such  activ- 
ity is  commonly  stopped  by  the  advent  of  the  arid  fore-summer,  and 


VEGETATION  OF  THE  SANTA  CATALINA  INIOUNTAINS.  25 

relatively  little  activity  is  observable  in  May  and  June,  at  least  away 
from  arroyos  and  springs.  In  the  Upper  Encinal  the  early  months  of 
spring  partake  of  the  rest  which  is  then  predominant  in  the  Forest 
region.  The  deciduous  trees  begin  foliation  in  late  April  or  early  May 
and  some  of  the  root  perennials  are  not  far  behind  them  in  their  earliest 
activity.  The  duration  of  the  arid  fore-summer  being  slightly  less  in 
the  Upper  Encinal  than  in  the  Lower  Encinal,  and  its  intensity  being 
also  less,  there  is  not  so  decided  a  break,  among  the  herbaceous  peren- 
nials between  the  first  activity  of  spring  and  that  of  the  humid  mid- 
summer, as  there  is  in  the  Lower  Encinal  and  the  Desert. 

THE  LOWER  ENCINAL. 

The  species  which  chiefly  characterize  the  Lower  Encinal  at  its 
desert  edge  have  already  been  mentioned :  Quercus  ohlongifolia,  Quer- 
cus  arizonica,  Juniperus,  Vauquelinia,  Dasylirion,  Nolina,  Yucca  macro- 
carpa,  Arctostaphylos  pungens,  Agave  palmeri,  and  Agave  schottii.  All 
of  these  are  much  more  abundant  at  5,000  feet  than  at  4,500  except 
Quercus  ohlongifolia,  which  is  a  tree  of  very  narrow  vertical  range,  rarely 
occurring  above  5,200  feet  and  reaching  its  limit  at  5,600  feet  on  steep 
south  slopes.  At  5,000  feet  the  Encinal  has  been  augmented  by  the 
appearance  of  the  common  trees  Quercus  emoryi  and  Pinus  cemhroides 
(pinon)  and  by  the  shrubs  Garrya  wrightii,  Mimosa  biuncifera,  Rhus 
trilohata,  and  Rhamnus  crocea  var.  pilosa  (see  plates  11a,  15,  and  16). 

The  only  characteristic  Desert  species  which  persist  throughout  the 
Lower  Encinal  are:  Carnegiea  gigantea,  a  single  young  individual  of 
which  has  been  seen  at  5,100  feet;  Opuntia  versicolor,  which  reaches 
5,500  feet;  Fouquieria  and  Echinocactus  wislizeni,  which  reach  5,600 
to  5,800  feet;  and  Mamillaria  grahami,  which  ascends  to  7,000  feet. 
So  far  as  known,  no  other  plants  occurring  on  the  Bajadas  or  in  any 
of  the  other  non-palustrine  desert  habitats  range  to  elevations  above 
6,000  feet.*  There  are  at  least  a  few  species  found  in  canons  and  near 
constant  water  which  range  from  the  elevation  of  the  Desert  to  more 
than  6,000  feet.  Several  of  the  typical  desert  genera  are  represented 
at  higher  elevations  by  species  which  seldom  range  as  low  as  the  Upper 
Desert  region.  Two  species  of  Opuntia  (platopuntias)  are  found 
throughout  the  Encinal,  growing  in  thin  soil  or  on  rocks,  and  reaching 
their  highest  occurrence  solely  on  ridges  or  upper  slopes.  One  of  these 
species  has  been  found  on  a  sharp  rocky  ridge  at  7,200  feet,  which  is 
the  highest  known  occurrence  of  a  platopuntia  in  the  Santa  Catalinas. 
Mamillaria  arizonica  ranges  from  the  Upper  Desert  to  nearly  7,000 
feet;  Echinocereus  polyacanthos  ranges  from  about  5,000  feet  to  7,800 
feet,  which  is  the  highest  elevation  at  which  any  cactus  has  been  found 
in  these  mountains.    Agave  palmeri  and  Yucca  schottii  are  also  fre- 

*  This  statement  is  made  only  with  respect  to  the  Santa  Catalinas.  The  influence  of  the 
character  of  the  underlying  rock  and  of  the  elevation  of  the  surrounding  desert  each  serves  to 
determine  indirectly  the  vertical  limits  of  desert  species. 


26  \t:getation  of  a  desert  mountain  range. 

quently  found  up  to  elevations  of  7,000  to  7,200  feet,  and  the  latter 
reaches  its  uppermost  limit  at  7,800  feet  (see  p.  30). 

The  ground  cover  of  low  perennial  plants,  grasses,  succulents,  and 
herbaceous  species  which  has  been  mentioned  as  characterizing  the  Upper 
Desert  is  Hkewise  to  be  found  throughout  the  Lower  Encinal,  but  does 
not  form  as  close  a  carpet  in  the  latter  region  as  it  does  in  the  former  (see 
plates  12,  15,  and  16).  Throughout  the  year  this  irregular  carpet  does 
much  to  lend  character  to  the  landscape,  varying  but  little  in  its  density 
with  the  alternating  seasons  of  vegetative  activity  and  of  drought  rest. 
The  scattered  polsters  of  Chrysoma  laricifolia  are  green  Sit  all  seasons,  and 
there  is  no  change  in  the  gray-green  foliage  of  Eriogonum  wrightii  nor 
in  the  white  tomentose  leaves  of  Artemisia  ludoviciana.  The  perennial 
grasses,  many  of  the  other  perennial  herbaceous  plants,  and  all  of  the 
ephemerals  are  either  in  a  resting  state  or  dead  throughout  the  arid 
fore-summer  and  the  arid  after-summer,  but  the  only  change  which 
their  rest  or  death  registers  in  the  landscape  is  a  change  of  its  color  tone 
from  a  greenish  gray  to  an  almost  uniform  gray  and  grayish  brown. 

All  of  the  low  shrubs  and  root  perennials  which  were  mentioned  as 
characteristic  of  the  Upper  Desert  are  to  be  found  occasionally  or 
commonly  in  the  Lower  Encinal,  excepting  Franseria  tenuifolia  and 
Ayenia  microphylla.  The  winter  and  spring  ephemerals  are  extremely 
few  at  4,500  to  5,000  feet,  but  there  is  much  activity  of  growth  and 
much  blooming  among  the  root  perennials  and  low  shrubs  during  the 
months  of  February  and  March,  and  sometimes  during  the  early  part 
of  April.  The  humid  mid-summer  is  a  season  of  even  greater  activity 
on  the  part  of  the  smaller  elements  of  the  vegetation.  Relatively  few  of 
the  conspicuous  herbaceous  plants  which  are  active  at  5,000  feet  in  the 
mid-sunamer  have  extended  upward  from  thebajada,  and  the  number  of 
summer  ephemeral  species  is  very  small  as  compared  with  the  Desert. 

Among  the  small  shrubs,  root  perennials  and  other  herbaceous  plants 
which  are  common  during  the  humid  mid-summer  at  4,500  to  5,500 
feet,  in  the  Lower  Encinal,  may  be  mentioned : 


Bacchans  pteronoides. 
Baccharis  thesioides. 
Bouteloua  hirsuta. 
Bouteloua  rothrockii. 
Castilleja  integra. 
Cordylanthus  wrightii. 
Croialaria  lupulina. 
Dalea  albiflora. 
Datea  mslizeni. 
Eriocarpum  gracile. 


Eriogonum  pharnaceoides. 
Euphorbia  heterophylla. 
Gilia  mulliflora. 
Gnaphalium  wrightii. 
Hymenothrix  wrightii. 
Linum  neomexicanum. 
Muhlenbergia  gracillima. 
Pappophorum  wrightii. 
Pentslemon  palmeri. 
Phaseolus  wrightii. 


On  the  flood-plains  and  along  the  streamways  of  the  Lower  Encinal 
may  be  found  a  greater  number  of  individuals  of  the  evergreen  oaks 
than  on  the  surrounding  slopes  (see  plate  10b),  and  also  Juglans  major, 
Platanus  wrightii,  and  Populus  sp.,  not  to  mention  the  restricted  occur- 
rence of  Cupressus  arizonica.     Shrubs  occasionally  found  along  the 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  27 

arroyos  are  Rhus  trilobata,  Baccharis  emoryi,  Erythrina  flabelliformis, 
Bouvardia  triphylla,  Amorpha  calif ornica,  Fendlera  rupicola,  Morus 
celtidifolia,  and  the  climber  Vitis  arizonica. 

On  the  flood-plains  of  the  larger  canons  in  the  Lower  Encinal  may- 
be found  the  lowest  examples  of  several  species  which  become  common 
in  the  forested  region  of  the  mountain,  and  these  are  indeed  the  lowest 
members  of  the  forest  flora,  if  aquatics  are  excepted.  At  4,900  feet 
Ceanothus  fendleri  and  Prunus  virens  are  both  to  be  found,  growing 
not  only  on  a  flood-plain  but  in  the  shade  of  evergreen  oaks.  Berberis 
wilcoxii  is  found  at  5,200  feet  growing  in  shade  near  a  constant  spring, 
and  Rhamnus  ursina  is  infrequent  at  5,000  feet  near  streamways. 

During  the  mid-summer  there  is  an  abundant  stand  of  herbaceous 
perennials  and  ephemerals  on  the  flood-plains  of  the  Lower  Encinal, 
giving  them  a  much  closer  carpet  of  vegetation  than  is  to  be  found  on 
the  adjacent  slopes.    Abundant  and  characteristic  among  them  are: 


Artemisia  sp. 
Asclepias  tuber osa. 
Brickellia  calijornica. 
Castilleja  Integra. 
ChamcBcrista  leptadenia. 
Comandra  pallida. 
Cordylanthus  wrighiii. 
Crolalaria  lupulina. 
Diodia  teres. 
Eriocarpum  gracile. 


Euphorbia  crenulata. 

Gyynnolomia  multiflora. 

Hymenothrix  wrightii. 

Malvastrum  sp. 

Monarda  pectinata. 

Solanum  douglasii. 

Solidago  sparsiflora  var.  subcinerea. 

Sporobolus  confusus. 

Stachys  coccinea. 

Stenophyllus  capillaris. 


THE  UPPER  ENCINAL. 

During  the  ascent  from  5,000  to  6,000  feet  the  most  notable  change 
in  the  vegetation  is  the  gradual  increase  in  the  density  of  the  stand  of 
evergreen  trees  and  shrubs  (see  plates  18,  19,  and  20),  a  change  which 
forms  the  chief  distinction  of  the  Upper  Encinal  from  the  Lower 
Encinal.  Quercus  emoryi  and  Quercus  arizonica  are  still  the  dominant 
trees,  while  Pinus  cembroides  and  Juniperus  pachyphlcea  are  somewhat 
less  common.  Arctostaphylos  pungens  and  Garrya  wrightii  are  the  most 
common  of  the  larger  shrubs  and  Mimosa  biuncifera  of  the  smaller  ones. 
Dasylirion  wheeleri,  Nolina  microcarpa,  and  Agave  palmeri  remain 
abundant,  at  least  on  southern  slopes,  up  to  6,000  feet  and  Agave 
schottii  remains  common  up  to  its  upper  limit  at  that  elevation.  With 
the  increasing  abundance  of  the  oaks,  however,  these  semi-desert 
species  as  well  as  the  cacti  become  infrequent  and  are  confined  to  the 
summits  of  ridges  and  the  crevices  of  rocks. 

On  steep  north  slopes,  between  5,300  and  6,000  feet,  many  almost 
pure  stands  of  Pinus  cembroides  are  to  be  found  and  also  the  lowest 
individuals  of  Quercus  reticulata,  here  a  low-branched  tree  of  20  feet 
in  height.  Pinus  chihuahuana  first  appears  at  about  5,900  feet  on 
south  slopes,  being  the  only  one  of  the  trees  which  is  not  found  at 
much  lower  elevations  on  north  slopes  than  on  south  ones — indeed  it 
is  not  common  on  north  slopes  at  any  elevation. 


28  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

The  heaviest  stands  of  the  Upper  Encinal  constitute  a  relatively- 
dense  thicket  in  which  the  trees  are  from  18  to  30  feet  in  height  and 
so  closely  placed  that  it  is  very  diflScult  for  a  mounted  man  to  make 
his  way  among  them.  This  is  partly  due  to  the  fact  that  the  oaks,  the 
juniper,  and  the  pinion  all  branch  freely  from  a  point  near  the  ground, 
and  partly  to  the  size  and  hemispherical  habit-  of  Arctostaphylos,  in 
which  many  of  the  stiff  branches  are  placed  in  a  nearly  horizontal 
position  near  the  ground.  These  dense  stands  of  the  Upper  Encinal, 
between  5,600  and  6,200  feet,  are  made  up  of  the  same  species  that 
form  the  very  open  Lower  Encinal  in  so  far  as  concerns  the  trees,  shrubs 
and  larger  perennials.  There  are,  however,  many  root-perennial  her- 
baceous plants  in  the  Upper  Encinal  which  are  not  to  be  found  below 
5,500  feet,  nearly  all  of  which  extend  upward  into  the  lower  portions 
of  the  Forest  region. 

Quercus  emoryi  is  still  a  common  tree  at  5,600  feet,  Quercus  arizonica 
is  replaced  by  the  closely  similar  Quercus  reticulata,  Quercus  hypoleuca 
makes  its  first  appearance,  and  Juniperus  pachyphlcea  and  Pinus  cem- 
broides  reach  their  maximum  abundance  between  5,500  and  6,500  feet. 
Dasylirion,  Nolina,  and  Yucca  are  still  conspicuous  elements  of  the 
vegetation  even  in  the  most  dense  stands  of  oaks,  but  Agave  schottii 
is  no  longer  found  and  Agave  palmeri,  Hke  the  cacti,  is  found  only  on 
ridges  and  rocks.  A  common  tree  of  the  lower  forest  region.  Arbutus 
arizonica,  is  first  found  in  the  Upper  Encinal,  where  its  isolated  indi- 
viduals are  conspicuously  different  from  the  oaks.  The  only  trees  of  the 
mountain,  excepting  the  desert  species,  the  ranges  of  which  lie  wholly 
below  the  Upper  Encinal,  are  Vauquelinia  calif  ornica  and  Quercus  oblong- 
ifolia,  while  Quercus  arizonica  reaches  its  upper  limit  in  this  region. 

Phoradendron  californicum,  the  mistletoe,  which  so  commonly  infests 
Prosopis  and  the  other  trees  of  the  desert,  is  found  throughout  the 
Desert  region  of  the  mountains,  while  in  the  Encinal  Phoradendron 
fiavescens  var.  villosum  is  found  on  several  hosts  and  Phoradendron 
juniperinum  is  extremely  common  on  Juniperus,  but  does  not  extend 
with  it  to  its  highest  occurrences. 

The  vegetation  of  the  Upper  Encinal  is  extremely  poor  in  shrubs 
of  the  type  so  common  in  the  Upper  Desert  and  still  frequent  in 
the  Lower  Encinal.  In  the  open  spots  there  may  be  found  a  few 
individuals  of  Artemisia  ludoviciana,  Parosela  wislizeni,  Anisolotus 
argensis,  and  other  dwarf  shrubs  of  the  Lower  Encinal,  while  in  the 
shade  of  the  heaviest  stands  of  oaks  are  to  be  seen  Pteris  aquilina  var. 
pubescens,  Muhlenbergia  affinis,  Polygala  alba,  Comandra  pallida, 
Hymenopappus  mexicanus,  Cordylanthus  wrightii,  Chenopodium  fre- 
montii,  and  other  species  of  root-perennials.  The  vegetation  of  rocks 
and  exposed  ridges  is  still  suggestive  of  the  desert,  both  in  its  physiog- 
nomy and  in  its  phyletic  relationships.  In  the  crevices  of  rocks,  where 
the  amount  of  soil  is  extremely  scant  and  the  supply  of  moisture  must 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  29 

be  very  uncertain,  Laphamia  lemmoni  is  found,  a  small  composite 
known  only  from  the  Encinal  region  and  only  from  this  habitat.  In 
crevices  more  favorably  situated  with  respect  to  moisture  may  be  found 
Heuchera  sanguinea,  and  on  north  slopes  at  6,000  feet,  in  moist  crevices, 
may  be  found  the  lowest  colonies  of  Saxifraga  eriophora,  a  plant  which 
occurs  infrequently  up  to  the  summit  of  the  mountain. 

Beds  of  Selaginella  rupincola  are  still  to  be  found  at  6,000  feet  and 
the  several  species  of  drought-resistant  ferns,  which  are  confined  to  the 
shade  of  rocks  at  lower  elevations,  are  conmion  on  the  floor  of  the  heavy 
stands  of  Pinus  cemhroides,  or  grow  among  the  boulders  in  more  open 
situations.  Among  these  the  most  common  are :  Cheilanthes  fendleri, 
Noiholcena sinuata,  Notholcenaferruginea,  and  Gymnopteris  hispida.  None 
of  these  species  extend  upward  into  the  Forest  region  (see  plate  14). 

The  drier  flood-plains  and  arroyos  of  the  Upper  Encinal  are  charac- 
terized by  the  same  oaks  and  evergreen  conifers  that  occur  on  the 
adjacent  slopes,  while  the  moister  stream  ways  bear  a  number  of  decidu- 
ous trees  and  shrubs,  notably  Juglans  rupestris  and  Platanus  wrightii, 
extending  upward  from  streamways  at  lower  elevations,  and  Prunus 
trirens,  Rhamnus  ursina,  Rhus  trilobata,  Robinia  neomexicana,  and  Rhus 
elegantula.  Less  frequent  are  Ceanothus  fendleri,  Berberis  wilcoxii,  and 
Bouvardia  triphylla,  and  Vitis  arizonica  is  still  common.  Pinus  chihua- 
huana  is  not  infrequent  along  the  drier  arroyos  at  the  lower  edge  of  its 
range,  and  Cupressus  arizonica  is  found  along  the  streams  and  on  the 
lower  slopes  of  Sabino  and  Bear  Canons  and  some  of  their  tributaries. 

The  commonest  herbaceous  perennials  of  the  flood-plains  of  the 
Upper  Encinal  are : 


Apocynurn  sp. 
Artemisia  dracunculoides. 
Asclepias  tuberosa. 
Carduus  rothrockii. 
Euphorbia  crenulata. 
Geranium  coespitosum. 
Gomphocarpus  hypoleucus. 
Gymnolomia  muUiflora. 
Monarda  pectinata  nutt. 
Muhlenbergia  sp. 


Oenothera  sp. 

Pentstemon  torreyi. 

Picradenia  biennis. 

Pteris  aquilina  var.  pubescens. 

Rubus  oligospermus. 

Senecio  neomexicanus. 

Solidago  sparsiflora  var.  subcinerea. 

Sporobolus  confusus. 

Thalictrum  fendleri  var.  wrightii. 

Zauschneria  calif  arnica. 


THE  FOREST  REGION. 

One  of  the  most  striking  changes  encountered  in  the  vegetational 
gradient  of  the  Santa  Catalinas  is  that  from  the  closed  and  relatively 
low  Encinal  to  the  open  forest  of  Pinus  arizonica,  with  trees  50  to  60 
feet  in  height.  This  pine,  the  Arizona  yellow  pine,  is  closely  related 
to  Pinus  ponderosa,  the  western  yellow  pine,  and  is  the  common  tree 
of  the  forested  altitudes  of  the  mountain,  extending  upward  on  south- 
erly slopes  to  the  summit  of  Mount  Lemmon.  The  lowest  stands  of 
pine  which  possess  sufficient  density  to  be  regarded  as  forest  occur  on 
northerly  slopes  at  5,800  to  6,000  feet,  or  on  southerly  slopes  at  6,000 
to  6,400  feet,  the  limits  depending  in  each  particular  locality  upon  the 


30  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

steepness  of  the  slope  and  its  soil  characteristics,  particularly  with 
respect  to  the  soil  moisture  supply  (see  plate  21). 

Much  more  gradual  and  inconspicuous  is  the  transition  from  the 
Pine  Forest  to  that  in  which  Abies  concolor  (white  fir)  is  the  dominant 
tree.  This  type  of  Forest  occupies  the  northern  slopes  of  the  highest 
summits  and  ridges  of  the  range  from  7,500  feet  upward,  but  there  are 
no  elevations  in  the  Santa  Catalinas  sufficiently  great  to  bring  the  Fir 
Forest  onto  the  south  slopes. 

Throughout  the  Pine  Forest  there  are  trees,  shrubs,  and  herbaceous 
plants  which  may  be  found  in  the  Encinal,  at  least  in  its  upper  portion, 
but  only  in  the  lowest  edge  of  the  Pine  Forest  may  plants  be  found 
which  suggest  the  genera  or  vegetation  types  characteristic  of  the  desert. 
A  single  cactus  (Echinocereus  polyacanthos) ,  a  Yucca,  and  an  Agave  are 
the  sole  representatives  of  the  succulent  and  semi-succulent  forms  of 
the  lower  elevations,  and  they  are  rare  above  7,000  feet  and  absent  above 
7,800  feet. 

The  Pine  Forest  is  not,  however,  without  vegetational  features  which 
suggest  the  effects  of  a  climate  not  far  removed  in  character  from  that 
of  the  desert.  The  openness  of  the  lowest  stands  of  Pinus  arizonica, 
the  high  mortality  among  the  seedlings  of  the  pine,  the  character  of 
the  foliage  of  the  shrubs  and  herbaceous  perennials,  and  the  deep-seated 
root  systems  of  the  latter  plants,  all  point  to  the  existence  of  a  pre- 
carious soil-moisture  supply  and  to  atmospheric  conditions  conducive 
to  active  transpiration.  In  the  Fir  Forest  none  of  these  features  is 
observable,  and  the  vegetation  as  a  whole  presents  a  much  more 
mesophilous  aspect. 

In  the  Forest  region  the  winter  is  a  season  of  almost  absolute  rest, 
save  for  the  photosynthetic  activity  which  is  doubtless  carried  on  by 
the  conifers,  and  possibly  by  the  evergreen  oaks  and  shrubs.  The 
deciduous  trees  and  shrubs  are  leafless  from  early  or  mid  October  until 
April  or  May,  and  only  a  few  herbaceous  perennials  are  active  during 
this  period,  such  as  the  evergreen  species  of  Pyrola  and  the  early  vernal 
plants,  such  as  Frasera.  The  amount  of  activity  on  the  part  of  the 
perennial  herbaceous  plants  during  the  arid  fore-summer  is  largely 
dependent  on  the  amount  of  winter  precipitation  and  the  date  of  its 
termination.  In  the  lower  portion  of  the  Pine  Forest  it  often  happens 
that  almost  all  activity  is  in  abeyance  until  the  first  rains  of  the  humid 
mid-summer,  while  in  the  upper  Pine  Forest  and  in  the  Fir  Forest  it 
is  always  possible  to  find  a  majority  of  the  common  herbaceous  plants 
in  activity  in  May  and  June.  There  is  a  notable  scarcity  of  annual 
plants  above  6,000  feet,  and  the  only  ones  that  have  been  detected  in 
the  Forest  region  are: 


Androsace  arizonica. 
Bidens  sp. 
Cerastium  sericeum. 
Dalea  polygonoides. 


Drymaria  sperguloides. 
Drymeria  tenella. 
Muhlenbergia  sp. 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  31 

THE  PINE  FOREST. 

In  the  lowest  stands  of  Pine  Forest  many  of  the  dominant  Encinal 
forms  are  still  to  be  found,  but  in  no  case  do  the  evergreen  oaks  fail 
to  become  more  and  more  scattered  in  occurrence  as  the  forest  of  pines 
becomes  more  dense.  Quercus  emoryi  and  Pinus  cembroides  are  scarcely 
concerned  in  the  overlapping  of  the  Chaparral  and  Forest,  as  the  former 
reaches  its  upper  limit  at  6,300  feet,  while  the  latter  becomes  confined 
to  the  rocky  non-forested  or  lightly  forested  ridges  at  about  the  same 
elevation,  although  it  persists  as  a  rare  shrub  to  an  elevation  of  7,800 
feet.  Arctostaphylos  and  Garry  a  are  likewise  of  infrequent  occurrence 
in  stands  of  forest.  The  oaks  which  are  characteristic  of  the  closed 
forest  are  Quercus  reticulata  and  Quercus  hypoleuca.  The  former  is  com- 
monly a  low-branching  shrub  which  often  forms  thickets  on  the  steep 
slopes  of  the  highest  peaks,  where  it  extends  upward  to  about  8,600  feet. 
The  latter  oak  is  a  shrub  near  its  lower  and  upper  limits  at  6,000  and 
8,500  feet  respectively,  but  attains  a  height  of  40  feet  and  a  girth  of 
3  to  4  feet  between  6,500  and  7,500  feet.  Juniperus  pachyphloea  is  of 
occasional  occurrence  in  the  Forest  up  to  7,900  feet,  and  Arbutus 
arizonica  (Arizona  madrona),  at  first  infrequent,  becomes  common  at 
7,000  to  7,500  feet  and  reaches  its  upper  limit  at  7,800  to  8,000  feet. 

The  composition  of  the  Forest  itself  is  extremely  simple  from  its 
lower  limit  around  6,000  feet  to  7,500  feet,  and  above  that  elevation 
is  equally  simple  on  southerly  slopes  up  to  the  summit  of  Mount 
Lemmon.  Pinus  chihuahuana  reaches  its  limit  at  about  6,700  feet  and 
forms  a  very  inconsiderable  portion  of  the  forest  throughout  the  upper- 
most 500  feet  of  its  vertical  range.  Pseudotsuga  mucronata  begins  to 
occur  on  steep  northerly  slopes  at  6,100  feet  and  Pinus  strobiformis 
(Mexican  white  pine)  at  6,800  to  7,000  feet,  but  neither  begins  to 
affect  the  composition  of  the  Forest  in  general  until  higher  elevations 
are  reached.  At  6,000  feet  the  streamways  and  flood-plains  are  char- 
acterized by  several  deciduous  trees  in  addition  to  the  pines  themselves. 
Platanus  wrightii  is  near  its  upper  limit  at  this  elevation,  Juglans 
rupestris,  Prunus  virens,  and  Acer  interior  are  of  frequent  occurrence, 
while  at  6,500  to  6,800  feet  are  found  the  lowest  individuals  of  Quercus 
submollis  and  Alnus  acuminata. 

Throughout  the  Pine  Forest  are  to  be  found  a  large  number  of  her- 
baceous perennials,  a  few  of  which  occur  in  the  Upper  Encinal,  the  great 
majority  of  which,  however,  accompany  the  closed  stands  of  pine,  with 
additions  and  eliminations  with  increasing  altitude.  In  addition  to  these 
plants  is  another  large  group  which  is  confined  in  occurrence  to  the  near 
proximity  of  streams  and  streamways;  some  of  the  members  of  the  group 
being  thus  restricted  in  occurrence  at  lower  altitudes,  while  they  are  of 
more  general  occurrence  on  heavily  wooded  slopes  at  higher  elevations. 

In  the  clear  park-like  stretches  of  Pine  Forest  where  no  evergreen 
oaks  happen  to  occur,  the  most  conspicuous  plants  on  the  forest  floor 


32 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


are  the  low  thorny  shrub  Ceanothus  fendleri,  or  varieties  of  it,  and  the 
bunch-grass  Muhlenbergia  virescens.  The  commonest  of  the  herbaceous 
perennials  are  low,  small  plants  such  as  Hedeoma  hyssopifolia,  Hous- 
tonia  wrightii,  Poa  fendleriana,  Calliandra  reticulata,  and  Calliandra 
humilis,  or  else  they  are  somewhat  taller  but  relatively  inconspicuous, 
as  Pseudocymopterus  montanus  var.  tenuif alius,  Erigeron  neomexicanus, 
Litliospermum  multiflorum,  Lotus  puherulus,  and  others.  In  the  dense 
shade  of  the  Upper  Encinal  Pteris  aquilina  var.  pubescens  is  common, 
and  it  again  becomes  common  in  the  pines  above  7,500  feet,  but  is  infre- 
quent in  the  lower  portion  of  the  forest  region. 

The  Pine  Forest  gives  the  impression  of  possessing  a  much  richer 
flora  of  herbaceous  plants  than  is  found  in  any  other  habitat  of  the 
mountain.  This  impression  is  due  to  the  fact  that  a  large  number  of 
species  enter  into  the  vegetation  as  very  coimnon  components  of  it. 
As  there  are  almost  no  rare  or  infrequent  species  to  be  found  in  the 
Pine  Forest  away  from  streams  and  springs,  the  total  flora  involved 
is  not  so  great  as  might  be  supposed  on  first  examination.  Following 
is  a  list  of  the  characteristic  species  found  between  7,000  and  8,000 
feet,  the  relative  abundance  of  which  is  indicated  by  asterisks: 


Characteristic  Herbaceoxis  Plants  of  the  Pine  Forest. 


Achillea  lanulosa. 
Agastache  pallidiflora. 
Anisololus  puherulus. 
Antennaria  marginata. 
Anthericum  torreyi. 
Apocynum  scopuloruyn. 
Bidens  sp. 

Brickellia  grandiflora. 
Calliandra  reticulata. 
Carpochcete  bigelovii. 
Castilleja  gloriosa. 
Cologania  longifolia. 
Commelina  dianthifolia. 
Desmodium  arizonicum. 
Desmodium  grahami. 
Dugaldia  hoopesii. 
Erigeron  macranthus. 
Erigeron  neomexicanus . 
Eupatorium  arizonicum. 
Eupatorium  pauper culum. 
Eupatoriuin  rothrockii. 
Geranium  ccespitosum. 
Gilia  thurberi. 
Gnaphalium  decurrens. 
Gnaphalium  wrightii. 
Gomphocarpus  hypoleucus. 
Hedeoma  hyssopifolia. 
Hieracium  discolor. 
Houstonia  wrightii. 
Hymenopappus  mezicanus. 
Ipomoea  muricata. 
Koeleria  cristata. 


Lathyrus  graminif alius. 

Lithospermum  multiflorum. 

Lupinus  sp. 

Microstylis  montana. 

Monarda  pectinata. 

Monarda  scabra. 

Muhlenbergia  virescens. 

Muhlenbergia  sp. 

Oenothera  hookeri. 

Onosmodium  thurberi. 

Panicum  bulbosum. 

Pentstemon  torreyi. 

Perityle  coronopifolia. 

Phaseolus  retusus. 

Pinaropappus  foliosus. 

Poa  fendleriana. 

Potentilla  subviscosa. 

Pseudocymopterus      montanus      var. 

tenuifolius. 
Pseudocymopterus      montanus      var. 

purpureus. 
Pteris  aquilina  var.  pubescens. 
Salvia  arizonica. 
Senecio  neomexicanus. 
Solidago  bigelovii. 
Solidago  marshallii. 
Stevia  sp. 

Tradescantia  pinetorum. 
Trifolium  pinetorum. 
Vicia  americana. 
Woodsia  mexicana. 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS.  33 

The  pure  or  nearly  pure  stands  of  Pinus  arizonica  which  occur  be- 
tween 8,000  and  9,000  feet  are  increasingly  poor  in  the  evergreen  oak 
shrubs,  which  have  disappeared  at  the  latter  altitude.  The  clumps  of 
young  Quercus  submollis  give  the  forest  its  only  deciduous  element  at 
this  altitude,  and  the  low  patches  of  Ceanothus,  so  conmion  at  8,000 
feet,  give  way  at  9,000  feet  to  Syinphoricarpos  oreophilus  and  to  the 
much  less  frequent  Holodiscus  dumosus.  Very  many  of  the  commonest 
herbaceous  perennials  of  the  Pine  Forests  which  lie  between  7,000  and 
8,000  feet  do  not  reach  9,000  feet,  or  are  replaced  in  the  physiognomy 
of  the  forest  by  closely  related  species.  On  the  summit  and  southern 
slopes  of  Mount  Lemmon  the  commonest  herbaceous  plants  are: 
Koderia  cristata,  Dugaldia  hoopesii,  Erigeron  neomexicanum,  Pteris 
aquilina  var.  pubescens,  Gnaphalium  decurrens,  Hieracium  lemmoni, 
Senecio  sp.,  Antennaria  marginata,  Silene  greggii,  and  Helianthella 
arizonica.  Throughout  the  higher  Pine  Forest  Arceuthobium  divari- 
catum  and  Arceuthobium  robustum  are  common  on  the  trunks  and  limbs 
of  Pinus  arizonica. 

At  about  6,800  feet  Alnus  acuminata,  Acer  interior,  and  Quercus 
submollis  become  frequent  along  streams  (see  plates  30  and  31).  The 
first  two  are  confined  to  this  habitat  throughout  their  vertical  range, 
while  the  oak,  which  is  the  only  deciduous  member  of  the  genus  in 
these  mountains,  is  found  even  in  some  of  the  driest  situations  above 
7,600  feet.  Quercus  submollis  occurs  characteristically  either  as  single 
trees  of  considerable  size,  up  to  40  feet  in  height  and  4  feet  in  girth, 
or  else  as  crowded  circumscribed  groups  of  young  trees,  which  doubt- 
less owe  their  juxtaposition  to  the  accidents  of  seed  dispersal.  Salix 
taxifolia  is  also  a  common  streamside  shrub  above  6,800  feet,  and 
in  certain  portions  of  the  mountain  Rosa  fendleri  is  abundant  in  the 
proximity  of  streams. 

Herbaceous  plants  are  to  be  found  in  increasing  numbers  at  or  near 
the  banks  of  streams  between  6,000  and  7,400  feet.  Prominent  among 
them  are:  J  uncus  arizonicus,  Aquilegia  chrysantha,  Thalictrum  fendleri 
var.  wrightii,  Scrophularia  sp.,  Trifolium  pinetorum,  Fragaria  ovalis, 
Potentilla  thurberi,  Hypericum  formosum,  Lobelia  gruina,  Agrimonia 
brittoniana  var.  occidentalis,  Gaura  suffulta,  and  Tagetes  lemmoni. 

THE  FIR  FOREST. 

Between  7,000  and  7,400  feet  is  a  rapid  change  in  the  character  of 
the  forest  stands  on  northerly  slopes,  due  to  the  increasing  occurrence 
of  Pseudotsuga  mucronata  and  Pinus  strobiformis,  the  lower  limits  of 
which  have  already  been  mentioned,  and  to  the  appearance  of  Abies 
concolor.  These  three  species  occur  in  mixed  stands  together  with 
Pinus  arizonica  on  northerly  slopes  up  to  about  7,500  feet,  above  which 
elevation  the  latter  becomes  a  very  infrequent  tree  on  slopes  facing 
directly  north,  although  it  still  occurs  in  admixture  with  Pseudotsuga 


34  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

and  Abies  at  9,000  feet  on  eastern  and  western  exposures.  Above  7,500 
feet  Pinus  strohiformis  ceases  to  be  confined  to  the  proximity  of  streams, 
and  occurs  in  admixture  with  Pseudotsuga  and  Abies,  but  is  not  so 
common  as  they  in  the  heaviest  stands  of  this  type  of  forest.  On  the 
north  slopes  of  Mount  Lemmon  is  a  small  colony  of  Abies  arizonica, 
which  is  not  known  from  any  other  locality  on  the  mountain. 

Slopes  of  due  south  or  southwestern  exposure  are  held  by  Pinus 
arizonica  up  to  the  summit  of  Mount  Lemmon  at  9,150  feet,  with  a 
slight  occurrence  of  Pseudotsuga  and  Pinus  strobiformis  above  8,000 
feet.  The  Pseudotsuga  and  Abies  forest  is  found  in  fine  development 
at  7,500  feet  on  steep  north  exposures,  and  reaches  its  maximum  devel- 
opment in  stature  and  size  of  the  trees  on  the  north  slopes  of  Mount 
Lemmon  at  8,500  to  9,100  feet  (see  plates  1  and  35).  The  altitude  of 
the  Santa  Catalina  Mountains  is  nowhere  sufficient  to  admit  of  the 
occurrence  of  extended  bodies  of  such  forest,  nor  of  their  existence  on 
southerly  slopes. 

In  the  Fir  Forest  the  last  relicts  of  the  Encinal  have  disappeared : 
Quercus  hypoleuca,  Quercus  reticulata,  and  Juniperus  pachyphlcea  are 
nowhere  to  be  found  in  association  with  Pseudotsuga  and  Abies, 
although  they  may  grow  very  near  them  on  opposed  slopes.  Arbutus 
arizonica,  which  is  more  common  in  the  Pine  Forest  than  in  the  Encinal, 
is  likewise  absent  from  the  Fir  Forest.  The  deciduous  Quercus  submollis 
and  the  widely  distributed  Populus  tremuloides  are  the  commonest 
of  the  subordinate  trees,  the  latter  often  becoming  dominant  over 
areas  of  an  acre  or  more  in  extent,  where  it  ultimately  gives  way  to 
conifers. 

The  floor  of  the  Fir  Forest  is  much  more  heavily  and  continuously 
shaded  than  that  of  the  densest  stands  of  pine,  a  circumstance  which 
is  of  great  importance  in  determining  the  nature  of  the  forest  reproduc- 
tion and  also  in  conditioning  the  character  of  the  shrubby  and  herba- 
ceous vegetation.  The  dense  shade,  the  heavy  litter,  and  the  high  humus 
content  of  the  soil  tend  to  preserve  its  moisture  throughout  the  arid 
fore-summer  (see  p.  61),  so  that  the  seedling  trees  and  other  plants  of 
these  situations  are  very  far  removed  from  the  desiccating  influences 
which  are  operative  in  the  open  Pine  Forest.  The  Fir  Forest  near  the 
summits  of  ridges  is  somewhat  more  open  than  that  which  is  found  on 
middle  and  lower  slopes,  and  this  difference  is  accompanied  by  a  dis- 
similarity in  the  herbaceous  flora  of  upper  and  lower  slopes.  On  the 
latter  may  frequently  be  found  communities  of  plants  which  differ  little 
in  their  specific  make-up  from  the  communities  which  occupy  flood- 
plains,  although  they  are  much  less  dense. 

The  heaviest  stands  of  Abies  and  Pseudotsuga,  like  most  heavy  conif- 
erous forests,  are  relatively  poor  in  both  shrubs  and  herbaceous  plants. 
A  few  of  the  shrubs  common  to  the  water-courses  are  to  be  found  also 
in  the  Fir  Forest,  such  as  Jamesia  americana,  Symphoricarpos  oreo- 


VEGETATION  OF  THE  SANTA  CATALINA  MOUNTAINS. 


35 


philus,  Ribes  pinetorum,  and  Ruhus  neomexicanus.  The  trifoliate 
maple,  Acer  glabrum,  also  occurs  locally  on  the  north  slopes  of  Mount 
Lemmon.  The  poverty  in  the  stand  of  herbaceous  species  on  the  floor 
of  the  Fir  Forest  is  contrasted  with  the  large  number  of  species  to  be 
found,  which  is  probably  not  so  great,  however,  as  the  number  charac- 
teristic of  the  open  Pine  Forest.  Most  common  are:  Bromus  richard- 
sonii,  Cystopterisfragilis,  Geranium  ccespitosum,  Frasera  speciosa,  Thalic- 
trum  fendleri  var.  wrightii,  Galium  asperrimum,  Smilacina  sessilifolia, 
Osmorhiza  nuda,  Disporum  trachycarpum,  Viola  canadensis  var.  rydhergii, 
Oxalis  metcalfii,  Fragaria  ovalis,  Trifolium  rusbyi,  and  Draha  helleriana. 
On  Abies  the  parasitic  Phoradendron  bolleanum  is  not  infrequent. 

The  banks  of  constant  and  intermittent  streams  and  the  narrow 
flood-plains  of  the  Fir  Forest  region  form  a  series  of  habitats  with 
closely  similar  physical  conditions  and  with  nearly  identical  vegetation. 
In  them  are  to  be  found  a  greater  abundance  and  variety  of  trees  and 
shrubs  than  occur  in  topographically  analogous  habitats  at  lower  ele- 
vations. Abies,  Pscudotsuga,  Pinus  strobiformis,  and  even  Pinus 
arizonica  occur  in  this  habitat.  Its  commonest  woody  plants,  however, 
are  those  which  do  not  occur  in  other  situations,  as  Alnus  acuminata, 
Acer  interior,  Acer  brachypterum,  Salix  scouleriana,  Salix  exigua,  Salix 
taxifolia,  Sorbus  dumosa,  Cornus  stolonifera  var.  riparia,  Jamesia  ameri- 
cana,  Sambucus  vestita,  Symphoricarpos  oreophilus,  Rubus  arizonicus, 
Ribes  pinetorum,  and  Salix  sp. 

In  this  same  series  of  habitats,  which  are  the  most  elevated  of  the 
moist  habitats  of  the  mountain,  is  the  most  dense  stand  of  herbaceous 
vegetation  that  occurs  on  the  Santa  Catalinas.  This  vegetation  is  rich 
in  species  and  varies  in  its  make-up  from  place  to  place  according  to 
the  amount  of  soil  moisture  present  and  according  to  the  openness  or 
shade.  In  the  following  Hst  are  given  the  characteristic  plants  of  these 
situations.  The  two  species  of  Mimulus  are  the  only  plants  invariably 
confined  to  the  immediate  proximity  of  water.  Such  plants  as  Dugaldia 
and  Agrimonia,  on  the  other  hand,  are  found  only  in  the  unshaded  flood- 
plains.  A  comparison  of  this  list  with  that  just  given  for  the  floor  of  the 
Fir  Forest  will  show  that  the  latter  habitat  has  few  distinctive  species. 

Characteristic  Herbaceous  Plants  of  Flood-Plains,  Stream  Banks,  and  Lower  Slopes  in  the 

Fir  Forest. 


*  Aconitum  columbianum. 
**  Aclcea  viridiflora. 

***  Agrimonia    brittoniana  var.   occiden- 
talis. 
**  Agrostis  scabra  var.  subrepens. 

*  Aralia  humilis. 

**  Aspidium  filix-mas. 
***  Bromus  richardsonii. 
***  Car  ex  sp. 

**  Carex  sp. 

**  Cerastium  sericeum. 

*  Delphinium  scopulorum. 


**  Disporum  trachycarpum. 

**  Draha  helleriana. 
***  Dugaldia  hoopesii. 

**  Epilobium  novomexicanum. 

**  Equisetum  robustum. 
***  Frasera  speciosa. 
***  Galium  asperrimum. 
*  Gentiana  microcalyx. 
***  Geranium  ccespitosum. 

**  Glyceria  nervata. 

**  Gyrostachys  sp. 

**  Heracleum  lanatum. 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


**  Humulus  lupidus  var.  neomexicanus. 
***  Hypericum  formosum. 
***  Juncus  hrunnescens. 
***  Juncus  interior. 

**  Limnorchis  sparsiflora. 
*  Ldstera  sp. 

**  Microslylis  porphyrea. 
***  Mimulus  cardinalis. 
***  Mimulus  guttalus. 
***  Osmorhiza  nuda. 
***  Oxalis  metcalfii. 

**  Oxalis  wrightii. 


*  Polygonum  douglasii, 

**  Pyrola  chlorantha. 

**  Pyrola  secunda. 

**  Rubus  arizonicus. 

**  Rudbeckia  laciniata. 
***  Scrophularia  sp. 
***  Smilacina  amplexicaulis. 

**  Smilacina  sessilifolia. 

**  Solanum  fendleri. 
***  Thalictrum  fendleri  var.  wrightii. 
***  Viola  canadensis  var.  rydbergii. 
***  Viola  nephrophylla. 


FLORA  OF  THE  SANTA  CATALINA  MOUNTAINS. 

The  wide  range  of  physical  conditions  embraced  within  the  area  of 
the  Santa  Catahna  Mountains  gives  them  a  relatively  large  flora,  which 
has  been  estimated  by  Professor  J.  J.  Thornber  to  be  about  1,500 
species.  Although  the  exploitation  of  this  flora  is  not  completed  it  is 
nevertheless  sufficiently  well  advanced  to  show  that  elements  are  pres- 
ent which  are  common  to  each  of  many  diverse  regions  lying  north, 
south,  east,  and  west. 

The  desert  at  the  foot  of  the  mountains  stands  in  unbroken  connec- 
tion with  the  deserts  of  Sonora  and  Sinaloa.  The  Encinal  and  Forest 
regions,  on  the  other  hand,  are  isolated  from  other  areas  possessing  the 
same  physical  conditions.  Areas  of  Encinal  are  numerous  and  near, 
both  on  the  low  desert  mountains  and  on  the  elevated  plains  of  southern 
Arizona;  while  bodies  of  forest  are  to  be  found  only  at  greater  distances 
and  more  remotely  separated  from  each  other.  The  floristic  history 
of  the  Encinal  and  Forest  regions  of  the  Santa  Catalinas  is  quite  as 
intimately  bound  up  with  the  controlling  influences  of  climatic  con- 
ditions as  is  the  present  limitation  of  the  vegetation.  In  fact  the  floras 
of  the  two  isolated  regions  are  a  resultant  between  the  physical  con- 
ditions which  they  have  presented  in  the  remote  and  recent  past  and 
the  operation  of  natural  agencies  of  dispersal. 

PHYTOGEOGRAPHIC  RELATIONSHIPS  OF  THE  FLORA. 

It  would  not  be  within  the  scope  of  this  paper  to  enter  upon  a  detailed 
discussion  of  the  floristic  relationships  of  the  isolated  mountain  areas 
of  Encinal  and  Forest  in  southern  Arizona,  even  if  all  the  evidence 
bearing  on  such  a  discussion  were  now  in  hand.  It  will  be  instructive, 
however,  to  point  out  very  briefly  some  of  the  principal  floristic  rela- 
tionships of  the  Santa  Catalinas  in  order  to  demonstrate  the  extensive 
and  diversified  area  over  which  members  of  its  flora  may  be  found. 

THE  DESERT  FLORA. 

The  flora  which  occupies  the  bajadas  of  the  Santa  Cruz  valley  and 
the  lower  slopes  of  the  Santa  Catalina  Mountains  derives  many  species 
from  each  of  two  Mexican  desert  regions,  the  one  lying  at  low  elevations 


FLORA  OF  THE  SANTA  CATALINA  MOUNTAINS.  37 

between  the  Sierra  Madre  Occidental  and  the  Gulf  of  California,  in  the 
States  of  Sonora  and  Sinaloa,  the  other  lying  at  higher  elevations  in 
the  States  of  Chihuahua  and  Zacatecas.  There  are  strong  diversities 
of  flora  between  these  two  Mexican  deserts,  although  they  do  not  fail 
to  have  many  species  in  common.  The  Sierra  Madre  forms  an  effective 
barrier  between  them  in  Mexico,  but  north  of  the  International  Bound- 
ary the  continental  divide  is  formed  by  scattered  mountain  ranges  and 
broad  valleys  rather  than  by  a  continuous  elevated  range,  and  these 
valleys,  lying  between  4,000  and  5,000  feet,  have  permitted  the  inter- 
mingling of  species  from  the  two  desert  floras,  at  the  same  time  that 
they  have  constituted  a  barrier  to  many  species  presumably  unable  to 
withstand  the  winter  temperature  conditions  of  the  elevated  valleys. 
The  deserts  which  border  the  lower  course  of  the  Colorado  River  in 
Arizona  and  California,  the  Mojave  Desert,  and  other  desert  regions 
in  southern  California  and  Nevada  lying  below  4,000  feet,  possess  a 
very  small  number  of  distinctive  species  as  contrasted  with  the  two 
Mexican  desert  regions,  and  have  contributed  almost  no  species  to  the 
flora  of  the  Santa  Cruz  valley,  although  many  species  of  wide  Mexican 
occurrence  are  represented  in  both  localities.  The  deserts  of  the  Great 
Basin  have  likewise  contributed  no  distinctive  elements  to  the  flora 
of  the  Santa  Cruz  Valley  and  the  Desert  region  of  the  Santa  Catalinas. 

Among  the  many  species  characteristic  of  the  Arizona-Sonora  Desert 
which  do  not  cross  the  continental  divide  are:  Carnegiea  gigantea, 
Parkinsonia  microphylla,  Encelia  farinosa,  Olneya  tesota,  Hyptis  emoryi, 
Franseria  deltoidea,  Simmondsia  calif ornica,  Jatropha  cardiophylla,  and 
Crossosoma  higelovii.  Among  the  desert  species  which  are  common  to 
the  Arizona-Sonora  region  and  to  the  Texas-Chihuahua  desert  are: 
Fouquieria  splendens,  Kceberlinia  spinosa,  Chilopsis  saligna,  Momisia 
pallida,  Coldenia  canescens,  Opuntia  leptocaulis,  Ephedra  trifurca, 
Hilaria  mutica,  and  Bailey  a  multiradiata. 

It  would  be  possible  to  place  perhaps  90  per  cent  of  the  desert  flora 
of  southern  Arizona  in  one  or  the  other  of  the  categories  just  mentioned. 
There  are  a  few  local  and  endemic  species,  but  very  few  species  exhibit 
ranges  extending  chiefly  to  the  west,  north,  or  east.  Among  the  two 
Mexican  elements  many  species  range  far  south  of  Mexico,  as  witness 
the  following,  which  are  found  in  the  deserts  of  Chile:  Calandrinia 
menziesii,  Bowlesia  lobata,  Daucus  pusillus,  Parietaria  dehilis,  and 
Hydrocotyle  ranunculoides.  A  large  number  of  the  genera  found  ia  the 
Desert  flora  also  possess  representatives  in  the  deserts  of  Argentine 
and  Chile,  as:  Covillea,  Franseria,  Encelia,  Actinella,  Krameria,  Gutier- 
rezia,  Viguiera,  Chorizanthe,  Coldenia,  Perezia,  Menodora,  Nama, 
Amsinckia  and  many  others.  Other  genera  found  in  the  Santa  Cruz 
Valley  have  many  representatives  in  tropical  South  America  or  in  the 
West  Indies,  as  Hyptis,  Dodoncea,  Erythrina,  and  Gymnolomia,  or  have 
a  world-wide  representation,  as  Tragia  and  Stemodia. 


38  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

Enough  has  been  said  to  show  that  both  the  specific  and  generic 
relationships  of  the  Desert  flora  are  with  the  desert  regions  of  Mexico, 
the  deserts  of  Argentine  and  Chile,  and  even  with  the  moist  tropical 
regions  of  South  America.  The  plants  which  dominate  the  Desert 
landscape  in  southern  Arizona  are  members  of  genera,  or  even  of 
species,  which  characterize  a  much  greater  area  to  the  south  than  to 
the  north,  and  they  are  in  the  main  members  of  genera  which  reach 
their  maximum  development  in  number  of  species  and  in  abundance 
of  individuals  in  similar  desert  regions.  The  plants  of  the  Desert  which 
are  of  tropical  relationship  are  usually  the  sole  and  northernmost  repre- 
sentatives of  families  or  genera  which  are  much  more  richly  represented, 
both  in  types  and  in  individuals,  in  the  tropical  zone.  These  plants 
are  often  so  infrequent  and  inconspicuous  as  scarcely  to  interest  the 
student  of  vegetation,  except  for  the  fact  that  their  seasonal  behavior 
and  habitat  relations  are  such  as  to  give  them  the  most  moist  conditions 
which  the  Desert  affords.  Among  them  may  be  mentioned :  Passiflora, 
Stemodia,  Maurandia,  and  Rivina. 

The  few  members  of  genera  of  northern  dominance,  such  as  Populus 
and  Salix,  or  Anemone  and  Delphinium,  are  either  to  be  sought  in  the 
vicinity  of  streams  and  ponds,  as  is  the  case  with  the  former  two,  or 
are  to  be  found  in  activity  only  in  the  late  winter  and  early  spring,  as 
is  true  of  the  latter  two.  The  still  fewer  species  of  transcontinental 
range  are  almost  solely  palustrine  plants,  as  Cephalanthus  occidentalis, 
Scirpus  americanus,  Cyperus  diandrus,  and  others,  and  are  to  be  found 
only  in  palustrine  situations  in  Arizona. 

THE  ENCINAL  FLORA. 

The  type  of  vegetation  which  is  designated  as  Encinal  in  this  paper 
is  found  throughout  southern  Arizona  and  New  Mexico  at  elevations 
of  5,000  to  7,000  feet.  It  is  pre-eminently  a  community  of  evergreen 
oaks  and  nut  pines,  with  many  sclerophyllous  shrubs.  With  many 
floristic  modifications  this  type  of  Encinal  extends  into  western  Texas, 
Colorado,  and  inner  California,  usually  as  a  belt  connecting  the  treeless 
plains  or  desert  with  the  forested  mountain  tops.  Encinal  similar  to 
that  of  southern  Arizona  is  found  throughout  the  mountainous  portions 
of  Sonora,  Chihuahua,  Sinaloa,  and  Zacatecas,  and  with  many  modi- 
fications it  extends  still  further  south. 

The  dominant  species  of  the  Encinal  of  the  Santa  Catalinas  range 
far  to  the  south  along  both  sides  of  the  Sierra  Madre,  whereas  but  few 
of  them  range  further  north  than  the  southern  edge  of  the  Mogollon 
Plateau  in  central  Arizona,  and  some  of  them  not  even  so  far  as  that. 
The  14  commonest  woody  or  semi-succulent  perennials  in  the  Encinal 
of  the  Santa  Catalinas  are  all  plants  of  extended  Sonoran  and  Chi- 
huahuan  distribution ;  all  of  them  occur  in  southern  New  Mexico  and 
eight  of  them  in  western  Texas.    Only  one  of  the  plants  reaches  Call- 


FLORA  OF  THE  SANTA  CATALINA  MOUNTAINS. 


39 


fornia  and  only  one  of  them  has  been  reported  from  Colorado.    These 
plants  are: 


Quercus  oblongifolia. 
Quercus  arizonica. 
Quercus  emoryi,  Tex. 
Vauquelinia  calif  or  nica. 
Juniperus  pachyphloea,  Tex. 


Arctostaphylos  pungens,  Cal. 
Garry  a  wrightii,  Tex. 
Dasylirion  ivheeleri,  Tex. 
Agave  palmeri. 
Nolina  microcarpa. 


Pinus  cembroides,  Tex. 
Mimosa  biuncifera,  Tex. 
Chrysoma  laricifolia,  Tex. 
Eriogonum  wrightii,  Col.,  Tex. 


The  Encinal  likewise  comprises  a  number  of  plants  which  reach  their 
maximum  occurrence  on  the  Great  Plains  or  else  possess  areas  of  dis- 
tribution which  are  chiefly  to  the  northeast  of  Arizona.  Among  these 
are  Bouteloua  obligostachya,  Bouteloua  hirsuta,  Bouteloua  curtipendula, 
Polygala  alba,  Artemisia  ludoviciana,  Artemisia  dracunculoides,  and 
Stephanomeria  runcinata. 

The  elements  which  are  common  to  the  flora  of  California  are  few, 
as  is  true  of  the  Desert,  and  are  almost  solely  comprised  in  the  follow- 
ing: Zauschneria  calif ornica,  Amorpha  calif ornica,  Bouvardia  triphylla, 
and  Brickellia  calif  ornica,  not  to  add  Arctostaphylos  pungens,  which  has 
its  maximum  extension  southward  into  Mexico.  The  Encinal  contains 
a  number  of  forms  which  have  been  but  recently  segregated  from  well- 
known  species,  among  them  Rhus  racemulosa,  Rhamnus  ursina,  and 
Prunus  virens.  So  little  is  known  of  the  ranges  of  these  species  that 
it  is  impossible  to  state  in  how  far  they  may  represent  contributions 
from  distant  floras  or  to  what  extent  they  represent  forms  that  have 
been  differentiated  in  the  Arizona-Sonora  region. 

The  only  northern  element  in  the  Encinal  flora  seems  to  be  that 
which  has  been  mentioned  as  occurring  also  in  the  Great  Plains,  while 
the  mountainous  regions  of  Colorado  and  Utah  have  contributed  even 
fewer  species  than  has  the  Calif ornian  region. 

THE  FOREST  FLORA. 

The  Forest  region  of  the  Santa  Catalinas  possesses  strong  floristic 
afl^inities  both  with  the  Mexican  cordillera  and  with  the  Rocky  Moun- 
tains of  Colorado  and  their  southern  extension  in  New  Mexico.  The 
majority  of  the  plants  which  take  a  conspicuous  place  in  the  vegetation 
of  the  Forest  are  members  of  northern  genera.  Many  of  these  members 
are  identical  with  Rocky  Mountain  species,  while  many  others  have 
their  chief  range  in  the  mountains  of  northern  Mexico.  There  are  also 
representatives  of  a  few  genera  which  are  distinctively  Mexican,  a  few 
species  of  northwestern  relationship,  and  a  few  apparently  of  restricted 
range  in  the  desert  mountains  of  Arizona  and  New  Mexico. 

As  examples  of  the  large  Rocky  Mountain  contingent  in  the  Forest 
flora  may  be  mentioned : 


Abies  concolor. 
Pseudolsuga  mucronata. 
Disporum  tr  achy  car  pum. 
Salix  scouleriana. 
Populus  tremuloides. 


Acer  glabrum. 
Jamesia  americana. 
Symphoricarpos  oreophilus. 
Frasera  speciosa. 
Dugaldia  hoopesii. 


Erigeron  macranthus. 
Heuchera  rubescens. 
Brickellia  grandiflora. 
Gilia  thurberi. 
Achillea  lanulosa. 


40  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

Some  of  the  members  of  this  group  are  of  wide  distribution  in  the 
north,  as  Populus  iremuloides,  Achillea  lanulosa,  and  Disporum  trachy- 
carpum.  In  the  northern  mountain  contingent  are  also  a  few  species 
which  range  eastward  to  the  Atlantic  coast,  a  few  which  are  found  at 
least  as  far  south  as  Maryland  {Heracleum  lanatum,  Rudbeckia  lacini- 
tata,  Apocynum  androscemifolmm,  Vicia  americana,  and  Asplenium  tri- 
chomanes),  not  to  mention  Achillea  lanulosa,  which  scarcely  deserves 
separation  from  the  cosmopolitan  Achillea  millefolium. 

The  relationship  with  northern  California  and  the  northwestern 
states  is  weakly  expressed  in  the  occurrence  of  Salix  lasiolepis  and 
Prunus  emarginata.  Genera  characteristic  of  the  sub-arctic  regions  are 
sparingly  represented  at  higher  elevations  by  species  of  Primula,  Saxi- 
fraga,  and  Androsace. 

Some  of  the  most  conspicuous  components  of  the  vegetation  belong 
to  northern  genera,  but  to  species  which  are  characteristic  of  the  Mexi- 
can Cordillera,  as  Pinus  arizonica,  Pinus  strobiformis,  Alnus  acuminata, 
Salix  bonplandiana,  Quercus  hypoleuca,  and  Quercus  reticulata.  Such 
genera  of  herbaceous  plants  as  Solidago,  Eupatorium,  Erigeron,  Pentste- 
mon,  Mimulus,  Potentilla,  Gilia,  and  Gentiana — all  of  which  are  richly 
developed  in  the  Rocky  Mountains — are  chiefly  represented  in  the 
Santa  Catalinas  by  species  not  found  in  Colorado  nor  Wyoming.  The 
extent  to  which  these  species  are  characteristic  of  the  Arizona-New 
Mexico  region  or  are  components  of  the  flora  of  the  higher  Mexican 
mountains  is  only  partially  known. 

The  relationship  of  the  Forest  flora  to  that  of  the  extended  mountain 
regions  to  the  south  is  still  further  strengthened  by  the  occurrence  of 
members  of  genera  which  are  not  found  in  the  Rocky  Mountains  of 
Colorado  and  northern  New  Mexico,  as  Arbutus,  Calliandra,  Micro- 
stylis,  Drymaria,  Cologania,  Stevia,  and  Tagetes. 

To  sunamarize  for  the  mountain  as  a  whole,  it  may  be  said  that  the 
floristic  relationships  of  the  Desert  and  Encinal  regions  are  almost 
wholly  with  the  Mexican  deserts  and  foothills  to  the  south,  while  those 
of  the  Forest  region  are  divided  between  the  Mexican  Cordillera  and 
the  Rocky  Mountains.  The  Mexican  group  is  the  more  conspicuous 
in  the  make-up  of  the  vegetation,  while  the  Rocky  Mountain  contin- 
gent is  apparently  preponderant  in  number  of  species. 

It  will  be  impossible  to  summarize  the  floristic  relationships  of  the  Santa 
Catalinas  in  a  thorough  manner  until  very  much  more  is  known  of  their 
own  flora  and  also  of  the  floras  of  the  many  adjacent  mountain  ranges  and 
desert  valleys,  both  in  the  United  States  and  in  Mexico.  For  the  explana- 
tion of  these  relationships  a  closer  acquaintance  is  needed  with  the 
actual  mechanisms  of  transport  which  are  effective  in  the  dispersal  of  the 
seeds  of  desert  and  mountain  plants.  A  fuller  knowledge  is  also  required 
of  the  fluctuations  of  climate  within  recent  geological  time,  and  of  the 
consequent  downward  and  upward  movements  of  the  Encinal  and  Forest 
belts  of  all  the  southwestern  mountains.    Such  movements  would  alter- 


FLORA  OF  THE  SANTA  CATALINA  MOUNTAINS. 


41 


nately  establish  and  break  the  connections  between  the  vegetations  of 
the  various  mountain  ranges  and  elevated  plains,  thereby  permitting  the 
dispersal  and  subsequent  isolation  of  species  which  might  find  no  means 
of  movement  across  the  desert  valleys  under  existing  conditions. 

LIST  OF  CHARACTERISTIC  SPECIES. 

The  lack  of  a  single  taxonomic  work  covering  the  entire  flora  of  the 
Santa  Catalina  Mountains  makes  it  desirable  to  bring  together  here  a 
list  of  the  plant  names  which  are  used  throughout  this  paper,  together 
with  some  of  the  commoner  synonyms.  The  list  comprises  only  those 
plants  which  are  common  and  characteristic  components  of  the  vege- 
tation of  some  particular  region  or  habitat  of  the  mountain.  The  writer 
wishes  to  express  here  his  very  great  indebtedness  to  Professor  J.  J. 
Thornber,  of  the  University  of  Arizona,  for  determining  numerous  sets 
of  plants  from  the  Santa  Catalinas  and  for  verifying  the  following  list. 


POLTPODIACE^: 

Aspidium  filix-mas  (L.)  Sw. 

=  Dryopteris  filix-mas  (L.)  Schott. 
Asplenium  trichomanes  L. 
Cheilanthes  fendleri  Hook. 
Cheilanthes  lindheimeri  Hook. 
Cheilanthes  wrightii  Hook. 
Cystopteris  fragilis  (L.)  Bernh. 

=  Filix  fragilis  (L.)  Underw. 
Gymnopteris  hispida  (Mett.)  Underw. 

=  Gymnogramme  hispida  Mett. 
Notholcvna  ferruginea  (Desv.)  Hook. 
Notholoena  hookeri  D.  C.  Eaton. 
Notholcena  sinuata  (Sw.)  Kaulf. 
Pelkca  wrightiana  Hook. 
Pteris  aquilina  var.  pubescens  Underw. 
Woodsia  mexicana  Fee. 
Equisetace^: 

Equisetum  robustum  A.  Br. 
Selaginellace^  : 

Selaginella  rupincola  Underw. 
Selaginella  sp. 

PlNACE^ : 

Abies  arizonica  Merriam. 

Abies  concolor  Lindl.  &  Gord. 

Cupressus  arizonica  Greene. 

Juniperus  pachyphlaea  Torr. 

Pinus  arizonica  Engelm. 

Pinus  cembroides  Zucc. 

Pinus  chihunhuana  Engelm. 

Pimis  strobiformis  Engelm. 

Pseudotsuga  mucronata  (Raf .)  Sudw. 
=  Pseudotsuga  taxifolia  (Lam.)  Britton. 
Gnetace^: 

Ephedra  trifurca  Torr. 
Gkamine.e: 

Agrostis  scabra  var.  subrepens  Hitchck. 

Andropogon  saccharoides  Sw. 

=  Amphilophis  saccharoides  (Sw.)  Nash. 

Andropogon  scoparium  Michx. 

Aristida  americana  var.  bromides  (H.  B.  K.) 
Scribn.  &  Merr. 

Aristida  divergens  Vasey. 

Aristida  scheidiana  Trin.  &  Rupr. 

Bouteloua  aristidoides  (Kunth)  Griseb. 

Bouteloua  curtipendula  (Michx.)  Torr. 


Gram  ine^ — Continued  ■ 

Bouteloua  hirsula  Lag. 

Bouteloua  oligostachya  (Nutt.)  Torr. 

Bouteloua  polystachya  (Benth.)  Torr. 

Bouteloua  rothrockii  Vasey. 

Bromus  richardsonii  Link. 

Diplachne  dubia  (Nees)  Benth. 

Eragrostis  lugens  Nees. 

Eragrostis  neomexicana  Vasey. 

Eragrostis  pilosa  (L.)  Beauv. 

Heteropogon  contortvs  (L.)  Beauv. 

Hilaria  cenchroides  H.  K.  B. 

Hilaria  mutica  (Buckl.)  Benth. 

Kccleria  cristata  (L.)  Pers. 

Leptochloa  mucronata  (Michx.)  Kunth. 

Muhlenbergia  affinis  Trin. 

Muhlenbergia  dumosa  Scribn. 

Muhlenbergia  distichophylla  (Presi)  Munro. 

Muhlenbergia  gracillima  Torr. 

Muhlenbergia  porteri  Scribn. 

Muhlenbergia  vaseyana  Scribn. 

Muhlenbergia  virescens  (H.  B.  K.)  Trin. 

Muhlenbergia  sp. 

Panicularia  nervata  (Willd.)  Kze. 

Panicum  bulbosum  H.  B.  K. 

Panicum  bulbosum  var.  minor  Vasey. 

Panicum  hallii  Vasey. 

Panicum  hirticaulum  Presl. 

Papphorum  wrightii  Wats. 

Poa  fendleriana  (Steud.)  Vasey. 

Sitanion  elymoides  Raf. 

Sporobolus  confusus  (Fourn.)  Vasey. 

Stipa  neomexicana  (Thurb.)  Scribn. 
Cypekace^: 

Carex  sp. 

Car  ex  sp. 

Cyperus  fendlerianus  Boeckl. 

Cyperus  inflexus  Muhl. 

Cyperus  speciosus  Vahl. 

Eleocharis  montana  (H.  B.  K.)  R.  &  S. 

Fi77ibristylis  sp. 

Hernicarpha  micrantha  (Vahl)  Britt. 

Stenophyllus  capillaris  (L.)  Britt. 

CoMMELINACEiE : 

Commelina  dianthifolia  DC. 
Tradescantia  scopulorum  Rose. 
Tradescantia  pinetorum  Greene. 


42 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


List  of  Characteristic  Species — Continued. 


JUNCACE^: 

Juncus  arizonicus  Wieg. 
Juncus  brunnescens  Rydb. 
Juncus  bufonius  L. 
Juncus  interior  Wieg. 

LlLIACE^ : 

Anthericum  torreyi  Baker. 

Broditea  capitata  var.  pauciflora  Wats. 

Calochortus  nuttallii  T.  &  G. 

Dasylirion  wheeleri  Wats. 

Disporum  trachycarpum  (Wats.)  B.  &  H. 

Nolina  microcarpa  Wats. 

Smilacina  amplexicaulis  Nutt. 

=  Vagnera  amplexicaulis  (Nutt.)  Morong. 
Smilacina  sessilifolia  Nutt. 
Yucca  macrocarpa  (Torr.)  Coville. 
Yucca  schottii  Engelm. 

AMARYLLIDACEiE : 

Agave  palmeri  Engelm. 
Agave  parryi  Engelm. 
Agave  schottii  Engelm. 
Iridace^: 

Sisyrinchium  arizonicum  Rothr. 

=  Oreolirion  arizonicum  (Rothr.)  Bicknell. 

ORCHIDACEiE : 

Gyrostachys  sp. 

Limnorchis  sparsiflora  (Wats.)  Rydb. 

Listera  sp. 

Microstylis  corymbosa  Wats. 

=  Achroanthes  corymbosa  (Wats.)  Greene. 
Microstylis  montana  Rothr. 

=  Achroanthes  montana  (Rothr.)  Greene. 
Microstylis  porphyrea  Ridley. 

=  Achroanthes  porphyrea  (Ridley)  Greene. 
Salicace^  : 

Populus  angustifolia  James. 
Populus  tremuloides  Michx. 
Popxdus  sp. 

near  to  Populus  wislizeni  (Wats.)   Sarg. 
Salix  bonplandiana  H.  B.  K. 
Salix  exigua  Nutt. 
Salix  scouleriana  Barr. 
Salix  taxifolia  H.  B.  K. 
Salix  wrightii  Anders. 
Salix  sp. 

JUGLANDACE^ : 

Jugland  major  (Torr.)  Hell. 
Betulace^  : 

Alnus  acuminata  H.  B.  K. 
=  Alnus  oblongifolia  Torr. 
Fagace^  : 

Quercus  arizonica  Engelm. 
Quercus  emoryi  Torr. 
Quercus  hypoleuca  Engelm. 
Quercus  oblongifolia  Torr. 
Quercus  reticulata  Humb.  &  Bonpl. 
Quercus  submollis  Rydb. 
Ulmace^: 

Momisia  pallida  (Torr.)  Planch. 

=  Celtis  pallida  Torr. 
Celtis  reticulata  Torr. 

=  Celtis  occidentalis  var.  reticulata  (Torr.) 
Sarg. 
Morace^: 

Humulus  lujmlus  var.  neomexicanus  Nels.  & 

Cockrl. 
Moms  celtidifolia  H.  B.  K. 


Santalace^: 

Comandra  pallida  A.  DC. 

LORANTHACE^ : 

Arceuthobium  divaricatum  Engelm. 

=  Razoumofskya  divaricate  (Engelm.)  Kze. 
Arceuthobium  robustum  Engelm. 

—  Razoumofskya  robusta  (Engelm.)  Kze. 
Phoradendron  bolleanum  Eichl. 
Phoradendron  californicum  Nutt. 
Phoradendron  flavescens  var.  villosum 

Engelm. 
Phoradendron  juniperinum  Engelm. 

POLYGONACE^: 

Chorizanthc  brevicornu  Torr. 

Eriogonum  abertianum  Torr. 

Eriogonum  pharnaceoides  Torr. 

Eriogonum  wrightii  Torr. 

Polygonum  douglasii  Greene. 

Rumex  hymenosepalus  Torr. 
Chenopodiace^  : 

Chenopodium  fremontii  Wats. 
Amarantace^  : 

Amaranthus  palmeri  Wats. 

Cladothrix  lanuginosa  Nutt. 

Frcelichia  floridana  (Nutt.)  Moq. 

Gomphreria  casspitosa  Torr. 

Gomphrena  nitida  Rothr. 
Nyctaginace^: 

Allionia  gracillima  Standley. 

Boerhaavia  pterocarpa  Wats. 

Boerhaavia  watsoni  Standley. 

Wedelia  incarnata  (L.)  Kze. 

PORTULACACE^: 

Calandrinia  menziesii  (Hook.)  T.  &  G. 

Calyptridium  monandrum  Nutt. 

Montia  perfoliata  (Donn.)  Howell. 

Talinum  patens  var.  sarmentosum  (Engelm.) 
Gray. 
Caryophyllace^  : 

Arenaria  confusa  Rydb. 

Cerastium  sericeum  Wats. 

Cerastium  texanum  Britt. 

Drymaria  sperguloides  Gray. 

Drymaria  tenella  Gray. 

Silene  laciniata  var.  greggii  (Gray)  Wats. 
Ranunculace^  : 

Aconitum  columbianum  Nutt. 

Actcea  viridiflora  Greene. 

Aquilegia  chrysantha  Gray. 

Clematis  ligusticifolia  Nutt. 

Myosurus  cupulatus  Wats. 

Thalictrum  fendleri  var.  wrightii  Gray. 
Berberidace^: 

Berberis  wilcoxii  Kearney. 
Papaverace^e  : 

EschschoUzia  mexicana  Greene. 

Platystemon  californicus  Benth. 
Crucifer^: 

Draba  helleriana  Greene. 

Draba  spectabilis  Greene. 

Lepidium  lasiocarpum  Nutt. 

Lesquerella  gordoni  (Gray)  Wats. 

Thelypodium  linearifolium  Gray. 
Crassulace^: 

Sedum  stelliforme  Wats. 

Tilloea  erecta  Hook.  &  Arn. 


FLORA  OF  THE  SANTA  CATALINA  MOUNTAINS. 


43 


List  of  Characteristic  Species — Continued. 


Saxifragace^  : 

Fendlera  rupicola  Engelm.  &  Gray. 
Heuchera  rubescens  Torr. 
Heuchera  sanguinea  Engelm. 
Jamesia  americana  T.  &  G. 

=  Edwinia  americaiia  (T.  &  G.)  Hell. 
Ribes  pinetorum  Greene. 
Saxifraga  eriophora  Wats. 

=  Micranthcs  eriophora  (Wats.)  Small. 
Platanace^  : 

Platanus  wrightii  Wats. 
Ceossosomatace^  : 

Crossosoma  bigelovii  Wats. 
RosacEvB: 

Agrimonia  brittoniana  var.  occidentalis  Bick- 

nell. 
Cowania  stansburiana  Torr. 
Fragaria  ovalis  (Lehm.)  Rydb. 
Hvlodiscus  dumosus  (Nutt.)  Hell. 
Potentilla  thurberi  Gray. 
Potentilla  subviscosa  Greene. 
Primus  virens  (Woot.  &  Stand.) 

=  Padus  virens  Woot.  &  Stand. 
Rosa  fendleri  Crepin. 
Rubus  arizonicus  Greene. 
Rubus  neomexicanus  Gray. 
Rubus  oligosperma  Thornb. 
Sorbus  dumosa  Greene. 
Vauquelinia  calif  arnica  (Torr.)  Sarg. 
Leguminos^: 

Acacia  greggii  Gray. 

Acacia  paucispina  Wooton. 

Acacia  suffrutescens  Rose. 

Amorpha  calif ornica  Nutt. 

Anisolotus  argensis  Coville. 

Anisoloius  puberulus  (Benth.)  Woot.  &  Stand. 

=  Hosackia  puberula  Benth. 
.An{soZo<usimpermus(Greene)Woot.&  Stand. 

=  Lotus  trispermus  Greene. 
Cassia  covesii  Gray. 
Cassia  leptadenia  Greenm. 

=  Chamcecrista    leptadenia    (Greenm.) 
Cockrl. 
Cassia  leptocarpa  Benth. 
Calliandra  eriophylla  Benth. 
Calliandra  reticulata  Gray. 
Calliandra  humilis  Benth. 
Cologania  longifolia  Gray. 
Crotolaria  lupulina  Raf. 
Dalea  albifiora  Gray. 

=  Parosela  albifiora  (Gray)  Vail. 
Dalea  parryi  T.  &  G. 

=  Parosela  parryi  (T.  &  G.)  Hell. 
Dalea  polygonoides  Gray. 

=  Parosela  polygonoides  (Gray)  Hell. 
Dalea  wislizeni  Gray. 

=  Parosela  wislizeni  (Gray)  Vail. 
Desmodium  arizonicum  Wats. 

=  Meibomia  arizonica  (Wats.)  Vail. 
Desmodium  bigelovii  Gray. 

=  Meibomia  bigelovii  (Gray)  Kze. 
Desmodiu7n  grahami  Gray. 

=  Meibomia  grahami  (Gray)  Kze. 
Desmodium  psilocarpum  Gray. 

=  Meibomia  psilocarpa  (Gray)  Kze. 
Erythrina  flabelliformis  Kearney. 
Eysenhardtia  orthocarpa  (Gray)  Wats. 
Indigofera  sphcerocarpa  Gray. 


LEGUMiNOSiE — Continued: 

Lathyrus  graminifolius  (Wats.)  White. 

Lupinus  sp. 

near  to  Lupinus  palmeri  Wats. 

Mimosa  biuncifera  Benth. 

Nissolia  schottii  (Torr.)  Gray. 

Parkinsonia  microphylla  Torr. 

Parkinsonia  torreyana  Wats. 

=  Cercidium  torreyanum  (Wats.)  Sarg. 

Phaseolus  retusus  Benth. 

Phaseolus  wrightii  Gray. 

Prosopis  velutina  Wooton. 

Robinia  neomexicana  Gray. 

Trifolium  pinetorum  Greene. 

Vicia  americana  Muhl. 

Vicia  melilotoides  Woot.  &  Stand. 
Geraniace^  : 

Geranium  ccespitosum  James. 

OXALIDACE^ : 

Oxalis  albicans  H.  B.  K. 

=  Ionoxalis  albicans  (H.  B.  K.)  Small. 
Oxalis  metcalfii  (Small). 

=  Io7wxalis  metcalfii  Small. 
LinacejE  : 

Linum  lewisii  Pursh. 
Linum  neomexicanum  Greene. 
Zygophyllace^  : 

Covillea  tridentata  (DC.)  Vail. 

=  Larrea  tridentata  (DC.)  Coville. 

RUTACE^ : 

Ptelea  cognata  Greene. 
Malpighiace^  : 

Janusia  gracilis  Gray. 
P0LYGALACE.E : 

Krameria  glandulosa  Rose  &  Painter. 

Poly  gala  alba  Nutt. 

EUPHORBIACE^ : 

Croton  texensis  (Klotsch)  Muell.  Arg. 
Euphorbia  crenulata  Engelm. 
Euphorbia  florida  Engelm. 
Euphorbia  heterophylla  L. 
Euphorbia  melanadenia  Torr. 
Euphorbia  pediculifera  Engelm. 
Jatropha  anguslidens  Muell.  Arg. 
Jatropha  cardiophylla  (Torr.)  Muel.  Arg. 
Manihot  carthaginensis  Muel.  Arg. 
Callitrichace^  : 
Callitriche  sp. 

Bu.KACE^ : 

Simmondsia  californica  Nutt. 

ANACARDIACE.E : 

Rhus  aromatica  var.  mollis  (Gray)  Ashe. 
Rhus  elegantula  Greene. 
Rhus  rydbergii  Small. 

=  Toxicodendron  rydbergii  (Small)  Greene. 
Rhus  trilobata  Nutt. 

ACERACE^: 

Acer  brachypterum  Woot.  &  Stand. 

Acer  glabrum  Torr. 

Acer  interior  Britt. 
Sapindace^: 

Dodonoea  viscosa    var.    angustifolia.    (L.  f.) 
Benth. 

Sapindus  drummondii  Hook.  &  Am. 
Rhamnace^  : 

Ceanothus  fendleri  Gray. 

Ceanothus  fendleri  var.  venosus  Trel. 

Rhamnus  crocea  var.  pilosa  Trel. 

Rhamnus  ursina  Greene. 


44 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


List  of  Characteristic 
"Rn/kUNACEM—Contimied : 

Zizyphus  lycioides  var.  canescens  Gray. 
=  Condalia  lycioides  (Gray)  Weberbaur. 

VlTACE^ : 

Parthenocissus     dumetorum     var.     laciniata 
Rehder. 
=  Parthenocissus  quinquefolia  var.  laciniata 
Planch. 
Vitia  arizonica  Engelm. 
Malvaceae: 

Abutilon  incanum  (Link)  Sweet. 
Ingenhoiisia  triloba  DC. 

=  Thurberia  thespesioides  Gray. 
Malvastrum  sp. 
Sphaeralcea  pedata  Torr. 
SterculiacejE  : 

Ayenia  microphylla  Gray. 
Htpericace^  : 

Hypericum  formosum  H.  B.  K. 
Fouquierace^: 

Fouquieria  splendens  Engelm. 

VlOLACE^ : 

Viola    canadensis    var.    rydbergii    (Greene) 


Viola  nephrophylla  Greene. 

LOASACE^ : 

Mentzelia  albicaulis  Dougl. 
Cactace^  : 

Carnegiea  gigantea  (Engelm.)  Britt.  &  Rose. 
=  Cereus  giganteus  Engelm. 

Echinocactus  wislizeni  Engelm. 

Echinocereus  fendleri  (Engelm.)  Rumpl. 

Echinocereus  polyacanthus  Engelm. 

Mamillaria  arizonica  Engelm. 

Mamillaria  grahami  Engelm. 

Opuntia  bigelovii  Engelm.  &  Bigel. 

Opuntia  blakeana  Rose. 

Opuntia  engelmanni  Salm  Dyck. 

Opuntia  fulgida  Engelm. 

Opuntia  Icevis  Coult. 

Opuntia  leptocaulis  DC. 

Opuntia  mamillata  Schott. 

Opuntia  santa-rita  (Griff.  &  Hare)  Rose. 

Opuntia     spinosior     (Engelm.     &     Bigel.) 
Tourney. 

Opuntia  toumeyi  Rose. 

Opuntia  versicolor  Engelm. 

Opuntia  sp. 

Opuntia  sp. 
Onagrace^: 

Epilobium  novomexicanum  Hausk. 

Gaura  suffulta  Engelm. 

Isnardia  palustris  L. 

=  Ludwigia  palustris  (L.)  Ell. 

(Enothera  hookeri  T.  &  G.  ;' 

=  Onagra  hookeri  (T.  &  G.)  Small. 

(Enothera  mexicana  Spach. 

Zauschneria  californica  Presl. 
Araliace^e  : 

Aralia  humilis  Cav. 
Umbellifer^: 

Daucus  pusillus  Michx. 

Heracleum  lanatum  Michx. 

Hydrocotyle  ranunculoides  L.  1. 

Osmorhiza  nuda  Torr. 

=  Washingtonia  obtusa  C.  &  R. 

Peeudocymopterus  montanus  var.  purpureus 
C.&R. 


Species — Continued. 
UMBELLIFER.E — Continued: 

Pseudocymopterus  montanus  var.  tenuifolius 
(Gray)  C.  &  R. 

CORNACE^: 

Cornus  stolonifera  var.  riparia  (Rydb.) 

Garrya  wrightii  Torr. 
Ericace^  : 

Arbutus  arizonica  (Gray)  Sarg. 

Arctostaphylos  pringlei  Parry. 

Arctostaphylos  pungens  H.  B.  K. 

Hypopitys  sanguinea  Hell. 

Pterospora  andromedea  Nutt. 

Pyrola  chlorantha  Sw. 

Pyrola  secunda  L. 
Primulace^: 

Androsace  diffusa  Small. 

Androsace  arizonica  Gray. 

Primula  rusbyi  Greene. 
Oleace^: 

Fraxinus  attenuata  Jones. 

Fraxinus  toumeyi  Britt. 

Menodora  scabra  Gray. 
Apoctnace^: 

Apocynum  androsmmifolium  L. 

Apocynum  scopulorum  Greene. 

Haplophyton  cimicidium  A.  DC. 

AsCLEPIADACEiE : 

Asclepias  linaria  Cav. 
Asclepias  tuberosa  L. 
Gomphocarpus  hypoleucus  Gray. 

CONVOLVULACE^ : 

Evolvulus  arizonicus  Gray. 

Ipomcea  capillacea  Don. 

Ipomoea  coccinea  var.  hederifolia  Gray. 

Ipomcea  muricata  Cav. 

POLEMONIACE^ : 

Gilia  floccosa  Gray. 
Gilia  multifiora  Nutt. 
Gilia  thurberi  Gray. 
Linanthus  aureus  (Nutt.)  Greene. 
Hydrophyllace^  : 
Ellisia  torreyi  Gray. 
Emmenanthe  pendulceflora  Benth. 
Nama  hispida  Gray. 
Phacelia  distans  Benth. 

BoRRAGINACEiE : 

Amsinckia  tessellata  Gray. 

Coldenia  canescens  DC. 

Cryptanthe  intermedia  (Gray)  Greene. 

Cryptanthe  pterocarpa  (Torr.)  Greene. 

Eremocarya  micrantha  (Torr.)  Greene. 

Lithospermum  muUiflorum  Torr. 

Onosmodium  thurberi  Gray. 

Pectocarya  linearis  DC. 
Verbenace^  : 

Lippia  wrightii  Gray. 

Verbena  ciliata  Benth. 

Verbena  wrightii  Gray. 
Labiate  : 

Agastache  pallidiflora  (Hell.)  Rydb. 

Hedeoma  hyssopifolia  Gray. 

Hyptis  emoryi  Torr. 

Monarda  pectinata  Nutt. 

Monarda  scabra  Beck. 

Salvia  arizonica  Gray. 

Stachys  coccinea  Jacq. 

Trischostema  arizonicum  Gray. 


FLORA  OF  THE  SANTA  CATALINA  MOUNTAINS. 


45 


List  of  Characteristic 

SOLANACE^ : 

Lycium  berlandieri  Dunal. 
Lycium  fremontii  Gray. 
Lycium  parviflorum  Gray. 
Nicotiana  trigonophylla  Dunal. 
Solanum  fendleri  Gray. 

SCKOPHULARIACE^  I 

Castilleja  gloriosa  Britt. 

Castilleja  integra  Gray. 

Cordylanthus  wrightii  Gray. 

Linaria  canadensis  L. 

Maurandia  antirrhinifolia  (Poir.)  Willd. 

Mecardonia  peduncularis  (Benth.)  Greene. 

Mimitanthe  pilosa  (Benth.)  Greene. 

—  Mimulus  pilosus  Wats. 
Mimulus  cardinalis  Dougl. 
Mimulus  guttatus  (L.)  DC. 
Mimulus  langsdorfii  Sims. 
Orthocarpus  purpurascens  Benth. 
Pentstemon  barbatus  (Cav.)  Nutt. 
Pentstemon  spectabilis  Thurber. 
Pentstemon  torreyi  Benth. 
Pentstemon  wrightii  Hook. 
Scrophularia  sp. 
Stemodia  durantifolia  (L.)  Sw. 

BlGNONIACE^: 

Chilopsis  linearis  (Cav.)  Sweet. 

=  Chilopsis  saligna  Don. 
Stenolobium  incisum  Standley. 

ACANTHACEiE: 

Anisacanthus  thurberi  (Torr.)  Gray. 

Carlowrightia  arizonica  Gray. 
Plantaginace^  : 

Plantago  fastigiata  Morris. 

Plantago  ignota  Morris. 
Rubiaceje: 

Bouvardia  triphylla  Salisb. 

Diodia  teres  Walt. 

Galium  asperrimum  Gray. 

Galium  rothrockii  Gray. 

Galium  wrightii  Gray. 

Houstonia  wrightii  Gray. 
Caprifoliace^: 

Sambucus  mexicana  Presl. 

Sambucus  vestila  Woot.  &  Stand. 

Symphoricarpos  oreophilus  Gray. 
ValerianacejE  : 

Valeriana  arizonica  Gray. 

CAMPANTJLACBiE : 

Lobelia  gruina  Cav. 

Specularia  biflora  (R.  &  P.)  Fisch.  &  Mey. 

COMPOSITiB : 

Achillea  lanulosa  Nutt. 
Actinolepis  lanosa  Gray. 
Antennaria  marginata  Greene. 
Artemisia  dracunculoides  Pursh. 
Artemisia  ludoviciana  Nutt. 
Artemisia  sp. 
Artemisia  sp. 
Baccharis  emoryi  Gray. 
Baccharis  glutinosa  Pers. 
Baccharis  pteronoides  DC. 
Baccharis  sarothroides  Gray. 
Baccharis  thesioides  H.  B.  K. 
Bwria  chrysostoma  Fisch.  &  Mey. 
Bahia  absinthifolia  Benth. 
Baileya  multiradiata  Harv.  &  Gray. 
Bebbia  juncea  (Benth.)  Greene. 


Species — Continued. 
Composite — Continued: 
Bidens  sp. 
Brickellia  californica  (T.  &  G.)  Gray. 

=  Coleosanthus  californicus  (T.  &  G.)  Kze. 
Brickellia  grandiflora  Nutt. 

=  Coleosanthus  grandiflorus  (Hook.)  Kze. 
Carduus  rothrockii  (Gray)  Greene. 
Carduus  sp. 

Carpochcete  bigelovii  Gray. 
Chrysoma  laricifolia  (Gray)  Greene. 

=  Aplopappus  laricifolius  Gray. 
Crassina  pumila  (Gray)  Kze. 

=  Zinnia  pumila  Gray. 
Dugaldia  hoopesii  (Gray)  Rydb. 
Encelia  farinosa  Gray. 
Erigeron  macranthus  Nutt. 
Erigeron  neomexicanus  Gray. 
Erigeron  wootoni  Rydb. 
Eriocarpum  gracile  (Nutt.)  Greene. 

=  Aplopappus  gracilis  (Nutt.)  Grey. 
Eupatorium  arizonicum  (Gray) . 

=  Eupatorium  occidentale  var.  arizonicum 
Gray. 
Eupatorium  pauperculum  Gray. 
Eupatorium  rothrockii  Gray. 
Franseria  cordifolia  Gray. 
Franseria  deltoidea  Torr. 

=  Gmrtneria  deltoidea  (Torr.)  Kze. 
Franseria  tenuifolia  Gray. 

=  GcBrtneria  tenuifolia  (Gray)  Kze. 
Franseria  ambrosioides  Cav. 
Gnaphalium  decurrens  Ives. 
Gnaphalium  wrightii  Gray. 
Gtiardiola  platyphylla  Gray. 
Gymnolomia  multiflora  Rothr. 
Gymnosperma  corymbosa  DC. 
Helenium  thurberi  Gray. 
Helianthella  arizonica  (Gray) . 

=  Helianthella  quinquenervis  var.  arizonica 
Gray. 
Hieracium  discolor. 
Hieracium  lemmoni  Gray. 
Hymenoclea  monogyra  T.  &  G. 
Hymenopappus  mexicanus  Gray. 
Hymenothrix  wrightii  Gray. 
Isocoma  hartwegi  (Gray)  Greene. 

=  Bigelovia  hartwegi  Gray. 
Laphamia  lemmoni  Gray. 
Laphamia  sp. 

near  to  Laphamia  halimifolia  Gray. 
Machcer  anther  a  tanacelifolia  (H.  B.  K.)  Nees. 
Pedis  papposa  Gray. 
Periiyle  coronopifolia  Gray. 
Picradenia  biennis  (Gray)  Greene. 

=  Actinella  biennis  Gray. 
Pinaropappus  foliosus  Hell. 
Psilostrophe  cooperi  (Gray)  Greene. 

=  Ridellia  cooperi  Gray. 
Rudbeckia  laciniata  L. 
Senecio  neomexicanus  Gray. 
Solidago  bigelovii  Gray. 
Solidago  marshallii  Rothr. 
Solidago  sparsiflora  var.  subcinerea  Gray. 
Stephanomeria  runcinata  Nutt. 
Stevia  sp. 

Tagetes  lemmoni  Gray. 
Trixis  angustifolia  var.  latiuscula  Gray. 
Verbesina  encelioides  (Cav.)  B.  &  H. 

=  Ximenesia  encelioides  Cav. 


46 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 

The  latitude  of  the  Santa  Catahna  Mountains  and  their  position  in 
the  midst  of  a  continental  desert  give  to  their  lower  slopes  the  cHmate 
which  is  well  known  to  characterize  southern  Arizona :  a  low  unequally- 
distributed  rainfall,  a  short  winter  with  frequent  severe  frost,  and  a 
long  summer  with  high  maximum  temperatures  and  low  atmospheric 
humidity.  The  longitudinal  position  of  the  Santa  Catalinas,  between 
the  Pacific  Coast  and  the  southern  Great  Plains,  gives  to  their  climate 
also  some  of  the  characteristics  of  both  these  regions,  notably  in  respect 
of  the  incidence  of  the  rainfall  seasons.  Both  the  winter  rains  of  the 
Pacific  Coast  and  the  summer  rains  which  are  prevalent  on  the  Great 
Plains  extend  in  attenuated  form  to  Tucson  and  to  the  Santa  Catalinas, 
giving  them  a  short  rainy  season  in  July  and  August,  often  extending 
over  into  September,  and  a  longer  less  pronounced  rainy  season  from 
December  to  February  or  March.*  Although  the  amount  of  rain  in  these 
seasons  increases  with  altitude,  the  duration  of  the  seasons  themselves 
is  essentially  the  same  from  Tucson  to  the  summit  of  Mount  Lemmon, 
and  in  fact  throughout  southeastern  Arizona. 


mm^:'t. 


m 


._L. 


M 


Fig.  2. — Schematic  representation  of  rainfall  seasons  and  length  of  frostless  season  at  Tucson 
and  in  the  Santa  Catalina  Mountains,  showing  averaged  limiting  dates  of  rainfall  seasons 
for  8  years  and  averaged  limits  of  the  frostless  season  for  1909,  1910,  and  1911  (A  A),  and 
for  1912,  1913,  and  1914  (B  B). 

The  long  frostless  season  characteristic  of  Tucson  and  the  foothills 
of  the  Santa  Catalinas  naturally  decreases  in  length  with  altitude  until 
at  8,000  feet  it  is  only  one  half  as  long.  The  curves  of  decreasing  length 
of  frostless  season  and  a  diagrammatic  representation  of  the  incidence 
of  the  rainy  seasons  are  shown  in  figure  2. 

The  gentle  rains  and  occasional  snowfall  of  the  winter  season  serve 
to  replenish  the  moisture  of  the  soil  at  all  altitudes,  but  on  the  desert 


*  See  Shreve,  Forrest. 
17:&-26,  1914. 


Rainfall  as  a  Determinant   of   Soil    Moisture.      The   Plant  World, 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  47 

their  effect  is  soon  overcome  by  the  desiccating  conditions  of  March . 
and  April.  The  hot  and  rainless  weeks  which  precede  the  mid-summer 
have  been  designated  the  ''arid  fore-summer."  On  the  desert  this  is 
a  season  in  which  the  temperature  conditions  are  conducive  to  activity 
on  the  part  of  plants,  while  the  soil  moisture  conditions  are  increasingly 
deterrent  to  it.  As  a  result  of  these  conflicting  conditions  activity  may 
be  observed  in  the  trees  which  grow  near  a  constant  water  supply,  as 
Populus  sp.  (cottonwood)  and  Salix  sp.  (willows),  trees  which  possess 
deep-seated  root  systems,  as  Prosopis  velutina  (mesquite),  and  plants 
which  contain  stores  of  water,  as  all  species  of  cacti.  The  activity  of 
Populus  and  Prosopis  consists  in  both  flowering  and  leafing-out,  as  well 
as  in  shoot  growth;  in  the  cacti  it  consists  in  flowering  and  in  some 
species  also  in  growth.  Among  all  desert  plants  other  than  those 
indicated  the  arid  fore-summer  is  a  period  of  drought-rest. 

With  respect  to  the  water  relations  of  plants  the  arid  fore-summer 
is  the  most  trying  season  of  the  year,  combining  low  soil  moistures  with 
atmospheric  conditions  that  compel  active  transpiration.  In  all  respects 
in  which  moisture  conditions  may  be  critical  for  the  survival  of  individuals 
or  the  limitation  of  the  distribution  of  species  it  is  in  the  arid  fore-summer 
that  the  critical  intensity  of  these  conditions  must  be  sought. 

The  retardation  of  spring  which  accompanies  increasing  altitude 
results  in  a  shortening  of  the  arid  fore-summer  from  a  length  of  15 
weeks  on  the  desert  to  11  weeks  at  6,000  feet  and  6  weeks  at  8,000  feet 
(see  fig.  2).  Not  only  does  this  trying  season  decrease  in  length  with 
altitude,  but  its  physical  conditions  become  amehorated,  as  will  be  shown. 

The  ''humid  mid-summer"  commences  on  July  8  and  lasts  until 
September  12,  these  being  the  average  dates,  for  8  years,  of  the  first 
and  last  rains  of  0.50  inch  or  more.  In  this  season  the  moisture  con- 
ditions of  desert  and  mountain  top  are  more  nearly  alike  than  at  any 
other  time.  It  is  the  season  of  greatest  vegetative  activity  on  the 
desert  and  in  the  forest  also.  On  the  desert  it  is  the  only  season  in 
which  germinations  take  place  among  the  perennials,  and  it  is  the  chief 
season  of  growth  among  all  perennial  plants,  including  those  that  have 
been  in  leaf  during  the  arid  fore-summer.  In  the  Encinal  region  the 
evergreen  oaks  renew  their  foliage  at  the  advent  of  spring,  but  the  great 
mass  of  vegetative  activity  awaits  the  humid  mid-summer.  In  the 
Forest  the  pines  also  commence  growth  with  the  cessation  of  frost, 
but  make  their  chief  growth  during  July  and  August.  The  humid 
mid-summer  is  also  the  chief  period  of  activity  for  the  herbaceous 
perennials  and  small  shrubs  of  the  forested  elevations.  Heavy  snow- 
fall during  mid-winter  or  the  occurrence  of  exceptionally  late  winter 
rains  may  bring  about  growth  among  the  herbaceous  perennials  of  the 
forest  during  the  arid  fore-summer.  In  fact  a  few  species,  notably 
Frasera  speciosa  and  Dugaldia  hoopesii,  commence  growth  before  the 
last  frosts  of  spring. 


48  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

At  the  higher  altitudes  the  shortness  of  the  growing  season  and  the 
coldness  of  its  nights  are  inimical  to  the  activity  of  the  herbaceous 
perennials.  These  circumstances  make  very  difficult  the  introduction 
into  the  Forest  region  of  plants  which  would  seem  calculated  to  flourish 
in  a  region  of  similar  moisture  conditions. 

After  the  close  of  the  humid  mid-smnmer  the  desert  is  subjected 
to  a  variable  period  of  6  to  10  weeks  of  arid  conditions,  a  season  known 
as  the  "arid  after-summer."  Although  the  temperature,  humidity, 
soil  moisture,  and  evaporation  may  reach  as  extreme  values  in  the 
arid  after-summer  as  in  the  arid  fore-summer,  nevertheless  the  total 
duration  of  such  extremes  is  not  as  great  in  the  former  season.  A 
general  cessation  of  vegetative  activity  occurs  in  September  and  Octo- 
ber at  the  higher  elevations  and  in  October  and  November  at  the  lower 
ones.  On  the  desert  it  sometimes  happens  that  occasional  rains  during 
the  arid  after-summer  prolong  the  activity  of  the  shrubs  and  even  of 
the  summer  ephemerals  to  such  a  late  date  that  they  may  be  seen  in 
flower  side  by  side  with  root-perennials  which  are  characteristic  of  the 
winter  season. 

RAINFALL. 

The  figures  for  the  monthly  average  rainfall  at  Tucson,  as  determined 
from  the  38-year  record  (1876  to  1913),  show  that  the  year  falls  natur- 
rally  into  two  humid  and  two  arid  seasons  (see  fig.  4).  Without  regard 
to  the  average  dates  upon  which  the  heavy  rains  of  the  humid  seasons 
commence  or  terminate,  the  humid  winter  may  be  seen  to  fall  within 
December,  January,  February,  and  March,  and  the  humid  mid-smnmer 
within  July,  August,  and  September.  Making  this  artificial  division 
by  months  between  the  rainfall  seasons,  the  percentages  of  the  total 
annual  precipitation  which  fall  in  the  four  seasons  are  as  follows: 
humid  winter  31.1  per  cent,  arid  fore-summer  5.9  per  cent,  humid  mid- 
summer 50.6  per  cent,  arid  after-summer  12.4  per  cent.  The  two  rainy 
seasons  yield  81.7  per  cent  of  the  total  annual  rainfall,  and  the  light 
rains  of  the  two  arid  seasons  (which  form  the  remaining  18.3  per  cent) 
are  of  very  shght  influence  upon  vegetation.  The  rains  of  November 
may  bring  forth  some  of  the  winter  herbaceous  perennials,  without  any 
effect  on  the  large  perennials  other  than  the  inducing  of  leaves  on  Fou- 
quieria  and  Parkinsonia.  The  rains  of  the  arid  fore-summer  are  usually 
too  light  and  too  widely  separated  to  bring  into  activity  either  the 
summer  ephemerals  or  the  perennial  plants. 

SEASONAL  DISTRIBUTION  OF  RAINFALL. 

On  the  Pacific  Coast  the  monthly  distribution  of  rainfall  brings  over 
75  per  cent  of  the  annual  total  within  the  winter  months.  On  passing 
eastward  through  Arizona  this  predominance  of  winter  rain  is  gradually 
lost  until  it  becomes  less  than  20  per  cent  of  the  annual  total  at  the 
Rio  Grande  River  in  New  Mexico.    Conversely,  the  precipitation  of 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


49 


the  summer  months  is  almost  negligible  on  the  Pacific  Coast  and  grad- 
ually increases  on  passing  eastward  until  it  reaches  50  per  cent  at  Tucson. 
Between  Tucson  and  the  Rio  Grande  it  remains  at  about  50  per  cent,  but 
from  the  basin  of  the  Rio  Grande  eastward  the  rainfall  seasons  of  the 
Tucson  region  cease  to  be  a  natural  division  of  the  year  (see  table  1). 

Table  1. — Percentages  of  summer  rainfall  and  of  winter  rainfall  to  the  annual  rainfall  for 
a  series  of  stations  stretching  from  the  Pacific  coast  to  the  Rio  Grande  River,  through 
southern  Arizona. 


Station. 

Winter. 

Summer. 

Total. 

76.0 
76.8 
73.3 
59.0 
47.5 
45.9 
43.7 
30.7 
24.8 
34.9 
28.3 
22.9 
17.6 

.7 
2.6 
12.0 
20.3 
35.5 
34.0 
37.6 
50.9 
57.5 
48.5 
49.9 
54.8 
66.8 

76.7 
79.4 
85.3 
79.3 
83.0 
79.9 
81.3 
81.6 
82.3 
83.4 
78.2 
77.7 
74.3 

Indio,  Cal 

Gila  Bend,  Ariz 

Benson,  Ariz 

Bowie,  Ariz. .  .    .          

Lordsburg,  N.  Mex 

Deming,  N.  Mex.    .    .  . 

Agricultural  College,  N.  Mex.   . . 

In  figure  3  are  given  curves  showing  the  percentages  of  the  annual 
rainfall  which  are  formed  by  summer  rains  and  by  winter  rains  for  a 
chain  of  13  stations  stretching  from  Los  Angeles  to  Mesilla  Park,  New 
Mexico,  on  the  Rio  Grande  River. 

Table  2. — The  total  annual  rainfall,  the  summer  rainfall,  and  the  percentage  of  the  latter  to 
the  former  for  very  wet  and  very  dry  years  at  Tucson. 


Years. 

Eight  wet  years 
(14  inches  or  over). 

Years. 

Eleven  dry  years 
(9  inches  or  less). 

An- 
nual. 

Sum- 
mer. 

Per- 
centage. 

An- 
nual. 

Sum- 
mer. 

Per- 
centage. 

1876 

14.02 
16.66 
14.92 
15.59 
15.07 
18.37 
15.04 
24.17 

10.18 
10.51 
11.98 
9.27 
1.77 
10.84 
9.04 
4.50 

72.6 
63.1 
80.3 
59.5 
11.7 
59.0 
60.1 
18.6 

1880           

6.61 
8.48 
5.26 
8.02 
7.78 
7.14 
8.38 
7.79 
8.61 
8.80 
7.85 

4.79 
3.03 
2.88 
3.97 
3.44 
2.61 
3.72 
2.45 
2.31 
5.36 
6.29 

72.5 
35.7 
54.8 
49.5 
44.2 
36.5 
44.3 
31.5 
26.8 
60.9 
67.4 

1878 

1883            

1881 

1885 

1882 

1886 

1884 

1891 

1889 

1894 

1890 

1899 

1905 

1900 

Average  percentage 

1902 

1903 

1904 

Average  percentage 

53.1 

47.6 

The  fall  of  approximately  half  the  annual  precipitation  in  the  humid 
mid-summer  is  by  no  means  a  constant  occurrence  at  Tucson.  In 
1881  the  summer  rainfall  was  80.3  per  cent  of  the  annual,  and  in  1884 


50 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


it  fell  to  11.7  per  cent.  Neither  does  the  percentage  of  summer  rain 
fluctuate  in  relation  to  the  occurrence  of  very  wet  or  very  dry  years. 
In  8  of  the  wettest  years  since  1876  (14  inches  or  above)  the  summer 
rain  was  53.1  per  cent  of  the  total,  and  in  11  of  the  driest  years  (9  inches 
or  less)  the  summer  yielded  47.6  per  cent  of  the  total  (see  table  2). 

The  impossibility  of  securing  figures  for  the  winter  precipitation  in 
the  Santa  Catalina  Mountains  makes  it  necessary  to  estimate  the 
annual  totals  of  rainfall  at  different  altitudes  from  the  known  figures 


Fig.  3. — Graphs  showing  percentage  of  winter  rainfall  to  annual  total 
(light  line) ,  and  of  summer  rainfall  to  annual  total  (heavy  line) ,  for 
a  chain  of  13  stations  from  the  Pacific  to  the  Rio  Grande. 

for  the  sununer  rain.  The  average  rainfall  at  the  stations  at  7,600  feet 
and  8,000  feet  for  the  years  1907  to  1914  is  17.45  inches  (443  mm.), 
from  which  it  may  be  assumed  that  the  annual  average  is  approximately 
35  inches  (889  mm.).  The  summer  rain  at  Tucson  during  1907  to  1914 
was  54.7  per  cent  of  the  annual  total.  If  the  seasonal  distribution  of 
rain  is  the  same  on  the  mountain  that  it  is  at  Tucson,  the  above  esti- 
mate of  the  annual  total  for  the  mountain  is  correct  within  1  or  2  inches. 
The  influence  of  altitude  on  the  seasonal  distribution  of  rainfall  in 
Arizona  is  a  matter  which  can  not  be  determined  without  further  data 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


51 


than  are  now  in  hand.  During  the  years  1907  to  1912  the  percentage 
at  Benson,  Arizona  (3,523  feet),  was  60  per  cent,  that  at  Globe,  Arizona 
(3,525  feet),  was  42.1  per  cent,  the  average  of  the  two  51.0  per  cent. 
The  average  of  the  percentages  for  Fort  Huachuca  (5,100  feet).  Fort 
Apache  (5,200  feet),  and  Bisbee  (5,500  feet)  is  55.5  per  cent.  Although 
these  figures  indicate  that  up  to 
5,000  feet  there  is  about  the  same 
percentage  that  holds  at  2,400  feet 
(at  Tucson),  nevertheless  at  Flag- 
staff (6,907  feet)  the  summer  rain 
was  only  40.7  per  cent  of  the  total 
in  the  years  mentioned.  At  Chlar- 
son's  Mill  (7,200  feet)  an  incom- 
plete record  indicates  that  in  1907, 
1909,  and  1910  the  summer  rain 
was  far  below  the  percentages  for 
Tucson  for  those  years.  At  Greer 
(9,200  feet),  on  the  Mogollon  Pla- 
teau, the  summer  rain  was  a  much 
greater  percentage  of  the  annual  total  in  1905  than  it  was  at  Tucson, 
while  in  1906  and  1908  the  percentages  were  nearly  identical.  It  can 
only  be  said,  therefore,  that  a  much  larger  body  of  data  is  necessary  to 
determine  the  possible  change  of  seasonal  distribution  of  rain  due  to 
altitude.  The  evidence  at  hand  indicates  that  there  is  little  probability 
of  a  marked  influence.     (See  table  3.) 

Table  3. — The  average  annual  rainfall  for  1907  to  1912,  the  average  summer  rainfall  for  the 
same  years,  and  the  -percentage  of  the  latter  to  the  former  for  stations  at  different  altitudes 
in  central  and  southern  Arizona. 


OCT      NOV     OEC. 


Fig.  4. — Diagram  showing  monthly  distribution 
of  rainfall  at  Tucson.  Averages  of  record  for 
38  years,  1876  to  1913  inclusive. 


station. 

Altitude. 

Annual. 

Summer. 

Percentage. 

Tucson 

Feet 
2390 

3523 
3525 

5100 
5200 
5500 

6907 

Inches 
11.54 

10.51 
17.01 

15.53 
16.95 
20.68 

22.60 

Inches 
6.66 

6.31 
7.16 

9.24 
7.53 
12.95 

9.20 

Inches 

57.7 

59.5  1 
44.4   [55.5 

62.6  J 

40.7 

Globe 

Fort  Huachuca 

Fort  Apache 

Bisbee 

Flagstaff 

ALTITUDINAL  INCREASE  OF  RAINFALL. 

The  measurements  of  summer  rainfall  on  the  Santa  Catalina  Moun- 
tains were  begun  in  1907  by  the  installation  of  a  metal  gauge  at  7,600 
feet,  where  the  record  was  secured  until  1911,  after  which  it  was  re- 
moved to  a  nearby  ridge  at  8,000  feet.  During  1908  and  1909  readings 
were  secured  at  the  base  of  the  mountain  and  at  6,000  feet,  in  1910  at 


52 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


6,000  feet  only.  In  1911  a  series  of  stations  was  selected  at  vertical 
intervals  of  1,000  feet,  from  the  base  of  the  mountain,  3,000  feet,  to 
the  station  at  8,000  feet,  and  in  1912  a  station  was  established  on 
Mount  Lemmon,  at  9,000  feet.  These  stations  have  been  continued 
in  the  succeeding  summers. 

The  readings  of  the  gauges  have  been  made  at  irregular  intervals,  as 
opportunity  afforded;  the  water  has  been  protected  from  evaporation 
by  the  use  of  kerosene,  and  has  been  measured  volumetrically.  The 
installation  of  the  gauges  has  been  made  each  spring  in  time  to  secure 
the  first  of  the  summer  rain,  and  the  final  readings  have  been  made  in 
September,  closing  in  1911  on  the  22d  to  the  25th,  in  1912  on  the  28th 
to  30th,  in  1913  on  the  25th  to  the  27th,  and  in  1914  on  October  10th 
to  11th.  The  location  of  the  gauges  at  the  various  altitudes  has  been 
such  as  to  give  them  comparable  topographic  surroundings.  Each 
station  is  at  the  summit  of  a  ridge  with  a  commanding  opening  to  the 
south  and  without  nearby  trees.  A  record  of  rain  has  also  been  secured 
at  the  Xero-Montane  Garden  at  5,300  feet,  near  the  head  of  Soldier 
Canon  and  just  below  the  6,000-foot  station.  A  recapitulation  of  all 
the  readings  of  mountain  rainfall  is  given  in  table  4. 


Table  4. — Summer  Rainfall  in  the  Santa  Catalina  Mountains. 

All  readings  cover  the  total  precipitation  of  July,  August,  and  September.  Starred  figures  include 
some  October  rainfall.  Figures  followed  by  plus  are  incomplete,  owing  to  the  overflowing  of 
gauges. 


Eleva- 
tion, 
feet. 

1907 

1908 

1909 

1910 

1911 

1912 

1913 

1914 

Averages  of 
perfect  records. 

Inches. 

Milli- 
meters. 

3,000 
4.000 
5,000 
6,300 
6,000 
7,000 
7,600 
8,000 
9,000 

6.65 

9.72 

6.27 
9.45 
11.97 
12.51 
11.07 
15.86 
21.30 

5.61 
9.77 

8.24 

8.67 

8.68 

14.57 

6.46 
8.59 
10.27 
6.09 
8.73 

10.62* 
14.73* 
19.13* 

'22^68*' 
27.64+ 

7.55 
10.63 
12.40 
8.88 
8.05 
15.21 
18.43 
16.47 
10.01 

192 
270 
315 
226 
204 
387 
468 
418 
254 

9.21* 
6.50 

10.75 
3.42 

6.05 
5.28 

20.92 

20.63 

17.91 

11.40 

19.76 
20.93  + 

13.18 
10.01 

27.82+ 
27.17+ 

The  only  record  of  daily  rainfall  for  the  Santa  Catalinas  is  one 
secured  in  Marshall  Gulch,  at  7,600  feet,  from  June  to  August  1911, 
by  Professor  J.  G.  Brown,  of  the  University  of  Arizona.  A  comparison 
of  the  daily  rainfall  at  Marshall  Gulch  and  at  8,000  feet  with  that  at 
the  Desert  Laboratory  (2,663  feet)  for  the  period  of  these  observations 
is  given  in  table  5.  The  number  of  rainy  days  on  the  desert  was  greater 
than  the  number  on  the  mountain  top — 31  and  19  respectively — owing 
to  the  16  days  with  only  a  trace  of  rain  at  the  Laboratory.  The  total 
rainfall  of  the  three  months  was  5.42  inches  at  the  Laboratory  (for 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


53 


exactly  the  same  days  covered  by  the  Marshall  Gulch  record),  and 
14.86  inches  at  the  mountain  station.  The  general  correspondence 
between  the  dates  of  heavier  rains  at  these  stations,  5,000  vertical 
feet  apart,  indicates  the  close  relationship  of  the  atmospheric  factors 
which  determine  the  rainfall  of  all  altitudes. 

Table  5. — Comparative  daily  incidence  of  rainfall  at  the  Desert  Laboratory  {2,663  feet)  and 
at  the  Montane  Garden  in  Marshall  Gulch  {7,600  feet),  for  June,  July,  and  August  1911. 


Day  of 
month. 

June. 

July. 

August. 

Day  of 
month. 

June. 

July. 

August. 

D.L. 

M.G. 

D.L. 

M.G. 

D.L. 

M.G. 

D.L. 

M.G. 

D.L. 

M.G. 

D.L. 

M.G. 

1st.   . 

.98 
T 

4.03 

17th .  . 

.19 
T 
T 
.03 
.21 

1.96 

.52 

2d 

18th .  . 

3d 

19th .  . 

.62 
.14 

.46 
.53 
.76 

.88 

lei' 

.10 

2.60 

.64 

4th 

20th .  . 

5th 

21st.. . 

6th 

22d... 

7th 

T 

'"!08' 

T 

'"!i5' 

.87 

23d.. . 

8th 

24th .  . 

.14 

9th 

T 
.10 
T 

25th    . 

.02 
.33 

10th.. 
11th. . 
12th.. 
13th.  . 
14th .  . 
15th.  . 

T 
.01 
T 
T 
T 

'!25' 

26th 

.42 
.20 

1.27 

T 

27th 

28th 

.42 
.02 

T 
T 
.10 

.30 

29th.. 
30th.. 
31st... 

T 
T 

16th. . 

.34 

Total  rainfall:  Desert  Laboratory,  5.42  in.;  Montane  Garden,  14.86  in.  Total  number  of 
rainy  days:   Desert  Laboratory,  15  (or  31,  including  days  with  T);   Montane  Garden,  19. 

Another  comparison  which  it  is  possible  to  institute  between  the 
summit  of  the  Santa  CataHnas  and  the  desert  is  the  summer  rainfall 
totals  from  1907  to  1914  inclusive  (see  fig.  9).  The  directions  of  the 
curves  which  show  the  march  of  the  summer  precipitation  from  year 
to  year  indicate  an  almost  complete  lack  of  relationship  between  the 
mountain  and  the  plain.  It  is  obvious  that  the  curve  of  altitudinal 
increase  of  rainfall  determined  in  such  a  year  as  1910  would  be  very 
unlike  the  curve  determined  in  1911. 

It  has  been  suggested  by  Smith  *  that  there  may  be  a  relative 
increase  of  rainfall  at  the  higher  altitudes  as  the  summer  advances, 
which  is  to  say  that  the  gradient  of  increase  of  rainfall  with  altitude  is 
steeper  for  the  late  summer  than  it  is  for  the  early  summer.  In  order 
to  test  this  possibility  the  series  of  ten  readings  taken  in  the  humid  mid- 
summer of  1911  and  the  one  set  taken  in  the  early  arid  after-summer 
have  been  grouped  into  totals  for  five  periods  of  approximately  one 
month  each  (table  6).  An  inspection  of  the  table  shows  that  the  maxi- 
mum rainfall  occurred  between  July  18  and  August  24  at  3,000,  4,000, 


*  Smith,  G.  E.  P.     Groundwater  Supply  and  Irrigation  in  the  Rillito  Valley. 
Exper.  Sta.  BuU.  64,  1910. 


Ariz.  Agric. 


54 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


5,000,  and  8,000  feet,  and  between  June  20  and  July  18  at  6,000  and 
7,000  feet.  In  similar  manner  the  less  frequent  readings  of  1912  and 
1913  have  been  divided  into  the  early  summer  and  late  summer  falls, 
by  the  latest  July  reading,  and  the  averaged  curves  for  early  summer 
and  late  summer  rain  are  of  nearly  the  same  shape,  but  the  late  sunamer 
curve  is  not  so  steep.  This  short  record  does  not  seem,  therefore,  to 
corroborate  the  suggestion  of  Smith. 

Table  6. — Intraseasonal  distribution  of  summer  rainfall  at  the  Desert  Laboratory  and  at  6 
elevations  in  the  Santa  Catalina  Mountains  for  1911. 


Rainfall  of  the  maximum  period 

in  heavy  type. 

Apr.  25-27 

June  20-21 

July  18-19 

Aug.  22-24 

Sept.  22-25 

station. 

to 

to 

to 

to 

to 

Totals. 

June  20-21. 

July  18-19. 

Aug.  22-24. 

Sept.  22-25. 

Oct.  12-14. 

Des.  Lab... 

0.01 

1.37 

.OS 

2.90 

1.61 

9.97 

3.000  feet... 

.00 

1.08 

3.39 

1.80 

1.46 

7.73 

4,000  feet... 

.00 

2.15 

4.65 

2.65 

1.69 

11.14 

5,000  feet... 

.00 

3.62 

5.40 

2.95 

2.75 

14.72 

6,000  feet... 

.00 

5.71 

3.45 

1.91 

1.92 

12.99 

7,000  feet... 

.00 

7.36 

3.68 

4.82 

2.16 

18.02 

7,600  feet... 

.69 

8.30 

8.83 

3.48 

2.56 

23.86 

The  increase  of  rainfall  which  accompanies  increase  of  altitude  is  a 
phenomenon  of  general  occurrence  throughout  the  southwestern 
United  States.  The  curves  by  which  such  increase  may  be  expressed 
differ  from  each  other  most  strikingly,  according  to  the  horizontal  dis- 
tance of  the  successive  stations  from  each  other,  according  to  the 
coastal  or  continental  position  of  the  series  of  stations,  or  according 
to  the  size  of  the  mountain  range  on  which  the  successive  elevations 
are  secured.  Although  it  is  possible  to  deduce  mathematical  formulae 
for  the  vertical  increase  of  rainfall,  it  is  necessary  to  introduce  into  all 
such  formulae  a  constant  for  the  particular  region  or  mountain  involved, 
and  the  figures  thus  secured  are  merely  in  the  nature  of  hj^Dothetical 
means  near  which  the  normal  conditions  may  fall.  It  would  be  of  very 
great  interest  in  the  extension  of  plant  geography  to  possess  data  on 
the  actual  amounts  of  rainfall  at  successive  elevations  in  a  large  number 
of  mountains  and  shelving  plains  throughout  the  southwest.  The  mean 
rainfall  conditions  which  are  expressed  in  a  gradient  based  on  a  long 
climatological  record  are  of  great  importance  in  connection  with  vege- 
tation, but  only  when  consideration  is  also  giveu  to  the  extremes  of 
rainfall,  and  particularly  to  the  lower  extremes,  if  a  semi-arid  country 
is  under  consideration.  The  securing  of  typical  normal  gradients  of 
altitudinal  increase  of  rainfall  is  not  of  so  much  importance  in  plant 
geography,  therefore,  as  a  knowledge  of  the  actual  oscillations  of  the 
rainfall  conditions  from  year  to  year  throughout  the  series  of  stations 
or  localities  involved. 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


55 


Smith*  has  deduced  two  curves  of  altitudinal  increase  of  rain,  one 
applicable  to  Pima  and  Pinal  Counties,  Arizona  (the  counties  in  which 
the  Santa  Catalinas  lie),  the  second  to  Graham  and  Cochise  Counties. 
These  curves  are  based  on  records  of  various  lengths,  chiefly  from  sta- 
tions located  in  the  valleys  of  these  mountainous  counties.  Smith's 
curves  are  reproduced  in  figure  5,  in  which  they  have  been  brought 
half  way  down  toward  the  base  hne  in  order  to  make  them  comparable 
with  the  curve  expressing  the  average  summer  rainfall  of  the  Santa 
Catalina  Mountains  for  1911,  1912,  and  1913.  The  portion  of  Smith's 
Graham-Cochise  curve  extending  above  5,500  feet  is  based  on  a  single 
short  record  at  6,000  feet. 

The  curve  of  altitudinal  rise  of  rain  for  1911,  1912,  and  1913  in  the 
Santa  Catahnas  is  merely  a  simple  average  of  the  actual  readings  for 
the  three  summers,  without  any  attempt  to  correct  in  accordance  with 


Fig.  5. — Graph  showing  altitudinal  increase  of  summer  rainfall  on  the  Santa  Catalina  Mountains 
in  1911, 1912,  and  1913  (solid  line) ;  together  with  Smith's  curves  for  Pima  and  Pinal  Counties, 
Arizona  (dotted  line),  and  for  Cochise  and  Graham  Counties  (broken  line). 

Fig.  6. — Graphs  showing  vertical  increase  of  summer  rainfall  in  the  Santa  Catahna  Mountains  in 
1911  (solid  line),  1912  (broken  line),  and  1913  (dotted  line). 

the  departure  of  the  neighboring  lowland  rainfall  from  the  normal 
during  these  years,  without  the  application  of  any  rainfall  formula, 
and  without  the  smoothing  of  the  lines.  Reference  to  table  4  will  show 
that  the  record  for  7,000  feet  is  based  on  two  years  only,  and  the  record 
for  9,000  feet  on  one  correct  summer's  reading  and  the  reading  of  one 
summer  in  which  the  gauge  overflowed. 

A  comparison  of  Smith's  curves  with  the  curve  for  the  Santa  Cata- 
linas shows  the  latter  to  have  a  sharper  rise  from  3,000  to  4,000  feet, 
and  to  have  a  relatively  level  stretch  from  4,000  to  6,000  feet,  where 
the  former  curves  have  their  sharpest  ascent. 


*  Smith;  G.  E.  P.,  loc.  cit. 


56 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


Figure  6  gives  the  actual  curves  for  the  three  summers  for  the  Santa 
CataUnas.  It  will  be  noted  that  in  each  curve  there  is  a  sharp  rise 
from  3,000  to  4,000  feet,  a  rise  which  continued  at  the  same  gradient 
to  5,000  feet  in  1911  and  1913.  From  these  submaxima,  reached  at 
5,000  feet  in  1911  and  1913  and  at  4,000  feet  in  1912,  there  is  a  fall  to 
a  subminimum  at  6,000  feet  in  the  two  former  years  and  at  5,000  feet 
in  the  latter  year.  There  is  then  a  pronounced  rise  in  the  curve  to 
7,000  and  8,000  feet.  The  rainfall  at  9,000  feet  in  1912  was  probably 
an  inch  or  more  greater  than  indicated  by  the  curve,  in  any  case  was 
greater  than  that  at  8,000  feet;  whereas  in  1913  the  precipitation  at 
9,000  feet  was  less  than  that  at  8,000  feet,  in  fact  less  than  that  at 
5,000  feet. 

The  horizontal  distances  between  the  rainfall  stations  were  unequal 
(see  plate  A),  the  angle  of  rise  from  3,000  to  4,000  feet  being  very 


Fig.  7. — Graph  showing  vertical  increase  of  summer  rainfall  in  the  Santa  Catalina  Mountains 

in  1911  (solid  line),  together  with  averaged  vertical  increase  in  a  series  of  13  Weather  Bureau 

stations  in  Arizona  (broken  line). 
Fig.  8. — Graph  showing  vertical  increase  of  summer  rainfall  in  the  Santa  Catalina  Mountains 

in  1912  (solid  line),  together  with  averaged  vertical  increase  in  a  series  of  21  Weather  Bureau 

stations  in  Arizona  (broken  line) . 

sharp,  that  from  4,000  to  5  000  slightly  less  sharp,  and  that  from  5,000 
to  6,000  still  less  sharp  and  exactly  equal  to  the  angle  of  rise  from  6,000 
to  7,000  feet.  The  stations  at  8,000  and  9,000  feet  are  located  at  the 
west  end  of  the  main  ridge  and  are  consequently  not  in  line  with  the 
lower  stations.  The  sharp  rise  in  elevation  between  the  3,000  and  4,000 
foot  stations  is  doubtless  partially  accountable  for  the  rapid  increase 
of  rainfall  between  them.  The  steep  rise  of  the  rainfall  graphs  between 
6,000  and  7,000  feet  may  indicate  an  influence  due  to  the  position  of 
the  7,000-foot  station  on  the  north  rim  of  Bear  Canon,  with  a  very 
abrupt  wall  immediately  below  it.  There  is  no  topographic  cause, 
however,  to  which  it  is  possible  to  attribute  the  dip  in  the  rainfall 
curves  for  6,000  feet  in  1911  and  5,000  feet  in  1912. 

In  order  to  institute  a  comparison  between  the  mountain  gradients 
of  rainfall  and  those  of  the  valley  stations  of  the  Weather  Bureau  the 
data  have  been  collated  which  are  expressed  in  the  curves  of  figures  7 


SHREVE 


Plate  A 


SANT>\ 


Cfi-O^ 


vAiL 


'''^>^/ 


n,.stBti< 


r%. 


2lL 


110°  40' 


IS  Geoiog.ca,  s  ,r«,      TOPOGRAPHIC  MAP  or  THE  SANTA  GAEATilNA  MOUNT.\m  S 


3'ir  mtervTil  1,000  feet 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  57 

and  8.  These  figures  compare  the  summer  rainfall  curves  of  the  Santa 
Catalinas  and  those  of  selected  stations  for  the  same  summers.  In 
figure  7  the  rainfall  of  July,  August,  and  September  1911  has  been  used, 
for  13  stations  located  in  southeastern  Arizona,  east  of  Phoenix  and 
south  of  Fort  Apache.  The  rain  has  been  averaged  for  each  group  of 
stations  lying  within  the  same  thousand-foot  interval  of  altitude. 
Figure  8  shows  the  curve  for  the  Santa  Catalinas  for  1912  and  the  curve 
for  21  stations  in  the  same  area.  A  single  record  above  5,000  feet  has 
been  available  for  this  curve,  that  at  Chlarson's  Mill,  in  the  Pinaleno 
Mountains. 

The  significance  of  the  comparison  of  these  rainfall  records  for  a 
single  season  is  entirely  different  from  that  of  averages  for  long  series 
of  years.  Such  a  comparison  as  this  makes  possible  the  contrasting  of 
records  which  are  strictly  contemporaneous  and  serves  to  show  the 
way  in  which  the  same  complex  of  meteorological  conditions  affected 
the  precipitation  at  various  altitudes,  and  how  these  conditions  affected 
the  rainfall  of  a  single  mountain  range  in  comparison  with  that  of  an 
extended  adjacent  area  lying  at  different  levels.  The  extremely  small 
number  of  rainfall  records  secured  at  localities  above  5,500  feet  in 
southern  Arizona  does  much  to  vitiate  such  a  comparison.  In  1911  the 
gradient  of  rise  was  greater  between  3,000  feet  and  5,000  feet  in  the 
Santa  Catalinas  than  it  was  in  the  Weather  Bureau  stations.  In  1912 
the  rise  was  sharper  in  the  mountains  from  3,000  to  4,000  feet  than 
it  was  in  the  valleys,  but  the  fall  from  4,000  to  5,000  feet  was  paralleled 
by  a  rise  in  the  curve  of  the  valley  stations.  The  fall  at  7,200  feet  at 
Chlarson's  Mill  was  far  below  that  at  8,000  feet  in  the  Santa  Catalinas 
for  the  same  period. 

The  shape  of  the  averaged  curve  of  rainfall  in  the  Santa  Catalinas 
for  the  three  summers  is  correlated  with  the  nature  and  movement 
of  the  convectional  storms  to  which  the  summer  precipitation  is  due. 
It  would  appear  that  certain  rains  are  derived  from  low-lying  clouds 
which  form  over  the  desert  and  are  then  driven  against  the  mountain 
wall  by  the  prevailing  southwest  winds  of  summer.  These  rains  in- 
crease in  intensity  as  they  pass  up  the  mountain  slopes  and  yield  their 
maximum  downpour  at  about  4,000  or  5,000  feet,  according  to  the 
conditions.  The  rainfall  at  the  Xero-Montane  Garden  was  greater 
than  that  at  the  6,000-foot  station  (700  feet  above  it  and  only  half  a 
mile  distant)  for  four  of  the  six  summers  in  which  records  have  been 
kept  in  the  two  localities  (see  table  4).  The  Garden  is  located  at  the 
head  of  Soldier  Canon,  and  just  above  it  there  is  a  sharp  increase  in 
the  gradient  of  the  mountain  slopes.  It  is  probable  that  the  head  of 
the  caiion  is  the  terminating  point  in  the  course  of  many  of  the  desert 
rain  storms.  The  rapid  increase  of  rainfall  between  6,000  and  7,000 
feet  may  be  due  to  a  similar  topographic  cause,  as  mentioned  in  a 
preceding  paragraph,  or  it  may  give  indication  that  the  rains  of  the 


58 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


higher  elevations  are  derived  from  a  higher  cloud  level,  probably  from 
convectional  clouds  which  form  at  times  when  the  atmospheric  con- 
ditions cause  condensation  at  a  greater  distance  from  the  earth.  When 
a  long  series  of  records  shall  have  been  secured  from  the  9,000-foot 
station  it  will  probably  show  that  its  average  rainfall  is  greater  than 
that  at  8,000  feet,  but  the  9,000-foot  record  for  1913  indicates  that 
there  will  be  occasional  years,  at  least,  in  which  the  maximum  for  the 
mountain  is  recorded  at  8,000  feet.  This  probably  means  that  at 
10,000  feet  on  adjacent  mountains  there  is  a  constantly  lower  rainfall 
than  at  8,000  or  9,000  feet. 

The  check  in  the  vertical  increase  of  rainfall  which  has  been  described 
as  occurring  between  4,000  and  6,000  feet  appears  to  be  absent  from 
all  curves  derived  from  widely  separated  valley  stations.  The  writer 
has  seen  no  such  plateau  in  any 
curves  derived  from  southwest- 
ern data,  but  there  is  always  the 
possibility  that  a  plateau  has 
been  smoothed  out  of  the  curves 
or  that  the  data  have  been  sub- 
jected to  the  influence  of  a 
straight-line  equation.  The 
character  of  the  increase  of  pre- 
cipitation with  altitude  in  a  sin- 
gle small  range  of  mountains  is 
no  more  a  special  case  than  is  the 
increase  in  a  widely  separated 
series  of  stations  in  any  locations 
whatsoever.  In  so  far  as  con- 
cerns the  study  of  meteorological 
dynamics,  such  a  mountain 
range  as  the  Santa  Catalinas  offers  exceptional  opportunities  for  investi- 
gation, and  much  more  might  be  learned  in  a  single  summer  of  intensive 
meteorological  study  on  its  slopes  than  could  be  ascertained  by  an  exami- 
nation of  records  of  rainfall  covering  a  period  of  a  thousand  years. 

As  regards  vegetation,  the  most  important  feature  of  the  study  of 
rainfall  conditions  is  the  determination  of  the  extremes  of  variation  in 
the  amount  and  seasonal  distribution  of  rain,  and  the  ascertaining  of 
the  effect  of  these  extremes  upon  the  conditions  of  soil  moisture.  Years 
of  heavy  precipitation  are  important  for  the  maintenance  of  the  forest 
which  clothes  the  higher  mountain  slopes  and  for  the  general  restora- 
tion of  the  supplies  of  soil  moisture  and  ground  water.  The  years  of 
low  precipitation,  and  especially  the  series  of  consecutive  years  with 
deficient  rainfall,  are  of  first  importance  to  the  vegetation  which  occu- 
pies the  Encinal  region  of  the  mountains.  During  such  years,  and 
particularly  during  the  arid  fore-summer  of  such  years,  the  lowest 


FiQ.  9 


Graph  showing  lack  of  relation  between 
summer  rainfall  at  Marshall  Gulch  (7,600  feet) 
and  at  Desert  Laboratory  (2,663  feet)  from  1907 
to  1914. 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  59 

individuals  of  all  Encinal  and  Forest  species  are  subjected  to  conditions 
of  water  supply  which  are  perhaps  below  any  conditions  that  have 
previously  occurred  during  their  lives,  or  are  surely — in  the  case  of 
perennials — the  most  trying  conditions  when  considered  in  the  light 
of  the  plants  having  grown  to  greater  size  and  heavier  water-demand 
than  during  the  dry  periods  of  their  earher  existence. 

SOIL  MOISTURE. 

It  is  obvious,  from  a  consideration  of  the  monthly  distribution  of 
rainfall  in  the  Santa  Catalinas,  that  at  all  elevations  there  are  annually 
two  periods  of  high  soil  moisture,  coinciding  with  the  humid  mid- 
summer and  the  humid  winter,  and  two  periods  of  decreasing  soil  mois- 
ture content,  coinciding  with  the  arid  fore-summer  and  arid  after- 
sunmier.  The  influence  of  the  earliest  rains  of  summer  and  winter  is 
quickly  exerted  in  an  elevation  of  the  soil  moisture,  but  at  the  close 
of  these  seasons  it  is  with  relative  slowness  that  the  soil  falls  to  low 
percentages  of  moisture,  particularly  at  the  highest  altitudes.  The 
minimum  moisture  content  of  the  year  is  usually  to  be  detected  just 
before  the  first  heavy  rain  of  the  humid  mid-summer,  but  the  content 
in  September  or  October  may  sometimes  be  quite  as  low. 

At  low  elevations  in  the  Santa  Catalinas  the  annual  march  of  soil 
moisture  may  be  expected  to  be  analogous  to  that  which  has  been 
described  by  the  writer  for  Tumamoc  Hill,  the  site  of  the  Desert 
Laboratory.*  Marked  differences  will  result  from  a  comparison  of  the 
two  localities,  however,  owing  to  the  difference  in  the  character  of 
the  soil.  The  very  fine  clay  of  Tumamoc  Hill  is  conservative  in  its 
changes  of  moisture  content,  both  with  respect  to  increases  and  de- 
creases of  moisture,  while  the  coarse  loam  found  at  the  lower  elevation 
in  the  Santa  Catalinas  possesses  a  greater  permeability  and  a  lesser 
holding  power.  The  soils  of  elevations  of  7,000  feet  and  more  are 
richer  in  organic  matter  than  those  of  the  Desert  and  Encinal  regions 
of  the  mountain,  and  are  doubtless  more  like  the  clay  of  Tumamoc 
Hill  in  the  smoothness  of  their  curves  of  change  in  moisture  content. 

The  few  readings  of  soil  moisture  content  that  have  been  made  were 
directed  toward  a  determination  of  the  soil  conditions  in  the  most  arid 
portion  of  the  year.  It  is  obvious  that  it  is  these  annual  minima  which 
are  of  the  greatest  importance  to  plants,  particularly  to  such  plants 
as  are  near  the  lowest  limit  of  their  vertical  occurrence.  Much  less 
interest  attaches  to  the  high  moisture  contents  which  might  be  found 
in  the  midst  of  the  rainy  seasons.  It  is  true  that  these  high  moistures 
are  the  ones  which  call  forth  general  vegetative  activity  and  condition 
the  appearance  of  ephemeral  plants  at  the  lower  altitudes.  It  is  like- 
wise possible  that  high  and  protracted  soil  moistures  may  be  of  some 
importance  as  a  limiting  factor  for  desert  species  at  the  upper  edges 

*  Shreve,  Forrest.      Rainfall  as  a  Determinant  of  Soil  Moisture.     The  Plant  World,  17 : 9-26, 1914. 


60  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

of  their  ranges.  It  was  impossible,  nevertheless,  to  secure  a  set  of 
soil  samples  in  the  humid  mid-summer  which  would  be  representative 
of  the  maximum  moisture  conditions  and  at  the  same  time  comparable 
for  the  various  altitudes.  A  set  of  samples  taken  at  the  same  interval 
after  a  rain  of  the  same  amount,  at  each  of  the  several  elevations, 
would  comply  with  the  requirements. 

All  samples  of  soil  for  moisture  content  were  taken  from  a  depth 
of  15  cm.  The  conditions  at  this  depth  are  of  importance  for  ephemeral 
herbaceous  plants  and  for  some  shrubs,  but  the  trees  and  larger  shrubs 
are,  of  course,  dependent  for  their  supplies  on  much  more  deep-seated 
bodies  of  soil.  The  rocky  character  of  the  substratum  means  that  the 
largest  perennial  plants  are  dependent  to  a  great  extent  upon  the 
moisture  contained  in  the  soil  which  occupies  the  crevices  of  the  rock 
in  situ.  It  is  particularly  noticeable  that  the  lowest  trees  of  the  Encinal 
region  grow  in  the  uppermost  part  of  talus  slopes  or  along  the  bottoms 
of  cliffs.  In  such  situations  it  is  doubtless  possible  for  the  roots  of 
these  trees  to  reach  soil-filled  crevices  which  are  fed  by  gravity  with 
the  water  of  large  veins  of  soil  above. 

The  samples  of  soil  were  seciu"ed  by  digging  with  a  hand  trowel  and 
transferring  quickly  to  bottles,  which  were  tightly  stopped,  and  then 
coated  over  the  stopper  with  vasehne.  The  soils  were  dried  in  the 
original  bottles  by  heating  to  100°  C.  until  they  showed  constant 
weight.  The  percentages  of  moisture  have  been  calculated  on  the  dry 
weight  as  unity.  The  physical  texture  of  all  samples  taken  was  very 
similar,  but  there  was  a  greater  amount  of  humus  in  those  from  the 
higher  elevations. 

Three  series  of  soil  samples  were  taken  at  various  times  to  determine 
the  conditions  prevaiHng  in  the  arid  fore-summer.  These  samples 
were  taken  at  1,000-foot  intervals,  from  the  vicinity  of  the  rainfall 
stations,  and  were  secured  in  pairs,  one  sample  being  from  a  south 
elope  and  one  from  a  north  slope.  The  localities  chosen  for  sampling 
were  typical  of  the  slopes  at  the  several  elevations,  and  in  every  case 
the  pair  of  samples  was  secured  in  the  midst  of  the  dissimilar  vegeta- 
tions which  occupy  the  opposed  slopes. 

On  April  27  to  29,  1911,  a  series  was  secured  from  3,000  feet  to  7,000 
feet  (see  table  7).  For  the  three  months  preceding  the  taking  of  these 
samples  there  had  been  only  Ught  and  infrequent  rains  over  the  sur- 
rounding region,  the  rainfall  of  the  mountains  themselves  for  this  period 
being  unknown.  At  Tucson  there  was  a  rainfall  of  0.28  inch  on  April 
2,  and  there  was  no  appearance  of  rain  on  the  mountains  after  that 
date.  On  June  9  to  11  another  series  of  samples  was  secured  at  the 
same  stations,  together  with  a  pair  from  the  station  at  8,000  feet. 
There  had  been  no  rain  between  the  securing  of  the  two  sets  of  samples. 

A  comparison  of  the  percentages  of  moisture  in  April  and  in  June 
shows  them  to  be  of  about  the  same  order  of  magnitude.    The  relative 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


61 


dryness  of  the  three  months  preceding  the  taking  of  the  first  set  of 
samples,  together  with  the  25  days  of  rainless  weather  just  preceding 
the  taking  of  the  samples,  had  reduced  the  moisture  of  the  superficial 
soil  to  an  amount  which  was  near  the  minimum  for  the  year,  as  repre- 
sented by  the  percentages  for  June.  The  percentages  for  June  were 
all  slightly  lower  than  those  for  April.  The  reading  of  5.2  per  cent  for 
the  south  slope  at  5,000  feet  in  April  is  undoubtedly  too  high.  The 
fall  in  moisture  on  the  north  slope  at  7,000  feet  from  9.2  per  cent  in 
April  to  3.2  per  cent  in  June  is  doubtless  significant  of  the  long  reten- 

Table  7. — Soil  moisture  in  the  arid  fore-summer  at  a  depth  of  15  cm.  on  north  and  south 
exposures  at  seven  altitudes  on  the  Santa  Catalina  Mountains. 


Elevation 

and 
exposure. 

Apr.  27  to  29, 
1911. 

June  9  to  11, 
1911. 

May  15  to  20, 
1914. 

Average  of 
the  three  de- 
terminations. 

3,000  south 

4,000  south 

4,000  north 

5,000  south 

5,000  north 

6,000  south 

6,000  north 

7,000  south 

7.000  north 

8,000  south 

8,000  north 

9,000  south 

9,000  north 

2.5 
2.7 
3.2 
6.2 
3.8 
2.3 
5.1 
3.1 
9.2 

2.3 
2.4 
2.8 
1.8 
1.3 
1.3 
2.3 
2.6 
3.2 
6.1 
8.1 

1.2 
1.0 
1.5 
2.4 
5.5 
1.9 
3.0 
2.1 
4.1 
8.8 

14.5 
9.4 

27.9 

2.0 
2.0 
2.5 
3.1 
3.5 
1.8 
3.5 
2.6 
5.5 
7.4 

11.3 
9.4 

27.9 

The  percentages  are  based  on  dry  weight. 

tion  of  moisture  derived  from  winter  rains,  characteristic  of  the  forested 
elevations.  In  June  the  north  slope  at  8,000  feet  had  fallen  to  a  shghtly 
lower  percentage  of  moisture  than  the  north  slope  at  7,000  feet  in  April. 

Between  May  15  and  20,  1914,  another  series  of  moisture  samples 
was  secured  at  the  same  locaHties  and  extended  to  the  9,000-foot 
station.  The  preceding  winter  had  been  slightly  below  the  average  in 
precipitation,  but  the  rainfall  for  March  had  been  above  the  average. 
At  the  time  of  the  taking  of  the  samples  there  had  been  no  rains  for 
six  weeks.  This  series  of  percentages  is  similar  to  those  secured  in 
1911,  and  the  three  may  be  taken  together  as  indicating  the  average 
soil  moisture  conditions  of  the  arid  fore-summer. 

A  significant  feature  of  all  three  of  the  series  of  moisture  determina- 
tions is  the  fact  that  there  is  no  appreciable  increase  of  soil  moisture 
up  to  an  elevation  of  7,000  feet,  beyond  which  elevation  there  is  a 
sharp  rise  in  the  percentages,  particularly  those  for  the  north  slopes. 
In  other  words,  so  far  as  the  superficial  soil  moisture  conditions  are 
concerned,  the  arid  fore-summer  carries  the  desert  up  to  the  lower 
limit  of  the  Pine  Forest. 

One  of  the  underlying  causes  of  the  importance  of  slope  exposure 
for  vegetation  is  revealed  in  a  comparison  of  the  percentages  of  soil 


62 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


moisture  for  north  and  south  slopes.    For  the  stations  at  4,000,  5,000, 

and  6,000  feet  the  percentages  for  the  two  slopes  scarcely  differ  by 

more  than  the  error  which  may  be  attributed  to  the  inequalities  of  the 

moisture  in  adjacent  bodies  of  soil.    Nevertheless,  in  all  but  two  cases 

there  is  a  slightly  greater  moisture 

content  on  the  north  slope  than  on 

the  south.   At  7,000  feet  the  difference 

between  the  two  exposures  becomes 

greater,  and  is  still  greater  at  8,000 

and  at  9,000  feet.    An  inspection  of 

the  averages  (see  fig.  10)  shows  that 

the  south  slope  at  7,000  feet  has  a 

lower  soil  moisture  than  the  north 

slope  at  6,000  feet.    The  south  slope 

at  8,000  feet,  however,  has  a  higher 

moisture  than  the  north  slope   at 

7,000  feet. 

The  fact  that  there  is  a  very  slight 
difference  between  the  soil  moisture 
of  north  and  south  slopes  at  lower 
elevations  and  a  greater  difference 
with  increasing  altitude  would  sug- 
gest that  there  might  be  a  more  pronounced  set  of  vegetational  phe- 
nomena resulting  from  slope  exposure  at  higher  elevations  than  at  lower 
ones.  This  is,  indeed,  the  case,  and  will  be  discussed  under  a  later 
heading  (see  p.  98). 

Table  8. — Soil  moisture  in  the  arid  fore-summer  and  arid  after-summer  at  a  depth  of  15  cm. 
on  north  and  south  exposures,  in  shade  and  open,  at  various  altitudes  on  the  Santa  Catalina 
Mountains. 


Fig.  10. — Graph  showing  vertical  increase 
of  soil  moisture  at  15  cm.  in  the  Santa 
Catalina  Mountains  on  north  slopes 
(heavy  line)  and  south  slopes  (light  line). 
Average  of  three  determinations  in  arid 
fore-  summer. 


Date. 

Elevation. 

Slope, 
exposure. 

Location. 

Moisture. 

May  27,  1910 

Do 

Feet 
5,300 
5.300 
5,300 
7,600 

7,600 

5,030 
4,600 
4,600 
6.300 
5,300 

South 
South 
North 
South 

South 

South 
South 
North 
South 
North 

Open 
Shade 
Shade 
Open,  base 

of  slope 
Open,  crest 

of  ridge 
Open 
Open 
Open 
Open 
Shaded 

2.1 
2.4 
3.8 

4.7 

3.9 
5.7 

2.2 
4.0 
2.9 
2.6 

Do 

May  29,  1910 

Do 

May  30,  1910 

Sept.  24,  1910 

Do 

Do 

Do 

In  the  summer  of  1910  several  samples  of  soil  were  taken  for  the 
determination  of  the  moisture  conditions  on  opposed  slopes  and  in 
shaded  and  open  soil,  as  well  as  on  the  top  and  at  the  base  of  a  slope. 
These  data  are  shown  in  table  8.    The  September  readings,  when  taken 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  63 

in  comparison  with  the  April,  May,  and  June  readings,  show  moistures 
of  about  the  same  amount,  indicating  that  the  after-summer  is  often 
a  season  of  as  great  soil  aridity  as  the  fore-summer. 

The  data  for  shaded  and  unshaded  soil,  both  in  May  and  September, 
corroborate  similar  determinations  made  on  Tumamoc  Hill  and  go  to 
show  that  in  the  arid  seasons  the  influence  of  shade  in  sustaining  the 
moisture  of  soil  is  so  slight  as  to  be  negligible.  The  influence  of  shade 
in  retarding  the  desiccation  of  the  soil  just  after  a  rain  is  not  without 
its  importance,  but  in  the  Desert  and  Encinal  regions  the  soil  in  the 
shade  of  trees  will  soon  reach  as  low  a  percentage  as  that  in  the  full  sun. 

EVAPORATION. 

It  has  been  frequently  pointed  out,  in  recent  botanical  literature, 
that  the  measurement  of  the  evaporative  power  of  the  air  affords  a 
concise  expression  of  the  combined  effects  of  temperature,  humidity, 
and  air  movement  in  so  far  as  these  factors  affect  the  loss  of  water  by 
plants.  The  obvious  importance  of  these  factors — and  consequently 
of  evaporation — in  the  environmental  complex  of  the  Santa  Catalinas 
led  to  the  early  installation  of  a  series  of  atmometers  (or  evaporimeters) 
at  several  elevations  in  these  mountains.  In  the  summer  of  1906  Dr. 
B.  E.  Livingston  secured  data  from  three  porous  cup  atmometers  at 
elevations  of  6,000,  7,500,  and  8,000  feet.*  In  1908  and  1910  the 
writer  installed  series  of  atmometers  at  five  elevations,  from  which 
readings  were  secured  which  are  not  sufficiently  complete  and  reliable 
to  be  worthy  of  publication.  In  1911  a  new  series  of  atmometers  was 
installed  at  the  six  rainfall  stations,  from  3,000  to  8,000  feet  inclusive, 
at  1,000-foot  intervals.  These  instruments  were  exposed  in  pairs,  on 
north  and  south  exposures,  and  were  operated  in  the  most  careful 
manner,  in  accordance  with  the  experience  of  the  two  preceding  years. 
The  atmometers  were  read  at  fortnightly  intervals,  or  nearly  so,  and 
at  each  reading  fresh  cups  were  installed.  The  actual  readings  were 
reduced  to  standard  by  the  use  of  an  average  between  the  original  and 
the  final  coefficients  of  correction.  Only  good  distilled  water  was  used, 
and  it  was  conveyed  in  tin  canteens  (rather  than  galvanized  iron  ones) 
from  which  the  resin  remaining  from  the  soldering  had  been  removed 
with  carbon  bisulphide.  The  bottles  used  for  the  atmometers  had  a 
capacity  of  1  gallon  at  the  lower  stations  and  of  2  quarts  at  the  higher 
stations,  such  ample  amounts  of  water  providing  against  the  possible 
danger  of  the  atmometers  going  dry.  The  stoppers  in  the  mouths  of 
the  bottles  were  made  very  tight,  to  prevent  the  cups  from  blowing 
loose,  but  were  provided  with  grooves  to  admit  air.  These  grooves 
were  stopped  with  loose  cotton,  to  prevent  the  entrance  of  ants,  and 
the  stoppers  were  covered  by  aprons  to  exclude  rain.  The  atmometers 
were  all  exposed  in  situations  such  that  they  received  full  insolation 

*  Livingston,  B.  E.    Evaporation  and  Plant  Habitats.    The  Plant  World,  11:    1-9,  1908. 


64 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


throughout  the  day,  except  in  the  ease  of  the  instrument  on  the  north 
slope  at  8,000  feet,  which  it  was  impossible  to  place  in  such  a  way  as 
to  avoid  a  slight  amount  of  light  shade  in  the  mid-morning  and  in  the 
mid-afternoon. 

Readings  were  secured  at  the  stations  from  3,000  to  7,000  feet  from 
April  25  to  27  until  September  5  to  6,  and  at  8,000  feet  from  June  7 
until  September  5.  The  actual  amounts  of  the  readings  are  given  in 
table  9,  in  terms  of  the  average  loss  per  day  in  cubic  centimeters  from 
a  standard  cup. 


Table  9.— The  average  daily  evaporation  (in  cubic  centimeters),  for  the  periods  indicated,  on 

north  and  south  exposures,  at  6  elevations  in  the  Santa  Catalina  Mountains. 

3,000 
feet 

4,000  feet. 

5,000  feet. 

6,000  feet. 

7,000  feet. 

8.000  feet. 

Dates. 

S 

s 

N 

s 

N 

S 

N 

S        N 

S 

N 

Apr.  25-27  to  May  18-19. 

97.2 

75.2 

68.6 

50.1 

51.2 

57.7 

52.4 

60.8  48.0 

.... 

May  18-19  to  June     6-  7 .  120 . 5 

84.8 

91.2 

74.2 

83.1 

67.7 

68.6 

72.5  57.5 

June     6-  7  to  June  20-22.     85.7 

81.3 

88.4 

60.8 

88.8 

52.8 

47.4 

55.2  44.3 

29.3 

29.4 

June  20-22  to  July     6-  7 . '  61 . 1 

67.6 

76.5 

50.9 

46.1 

50.4 

43.3 

46.8  43.3 

27.5 

16.9 

July     6-  7  to  July    18-21 .    43 . 2 

41.0 

43.7 

32.5 

25.5 

22.6 

24.5 

22.8  19.5 

10.7 

5.9 

July   18-21  to  Aug.     8-  9  J  59 . 8 

54.7 

53.5 

44.2 

37.2 

34.2 

33.6 

37.3  34.1 

23.3 

19.0 

Aug.     8-  9  to  Aug.  22-23. 

61.0 

56.0 

59.3 

45.2 

44.2 

37.6 

36.1 

39.9  27.0 

13.1 

7.4 

Aug.  22-23  to  Sept.    5-  6. 

55.6 

42.8 

56.9 

50.8 

33.4 

39.5 

28.3 

39.324.0 

18.9 

12.6 

Sept.    6-  6  to  Sept.  22-25. 

45.4 

56.9 

39.9 

31.6 

20.2 

22.5 

20.8 

28.0  30.4 

11.2 

6.8 

In  order  to  ascertain  the  altitudinal  gradient  of  evaporation  rate 
the  readings  from  the  north  and  south  slopes  at  each  altitude  have 
been  averaged.  The  averaged  total  evaporation  of  the  summer  for 
each  station  has  been  subdivided,  so  as  to  show  the  amount  for  the 
arid  fore-summer  as  shown  by  the  first  three  series  of  readings,  and 
for  the  humid  mid-summer  as  shown  by  the  last  six  series.  The  curves 
in  figure  12  show  the  altitudinal  fall  in  evaporation  rate  during  the 
two  seasons,  in  terms  of  the  average  daily  loss  from  the  atmometer. 
These  curves  bring  out  in  striking  manner  the  low  evaporation  rates 
of  the  humid  mid-summer  as  contrasted  with  the  arid  fore-summer, 
the  latter  being  nearly  twice  as  great  as  the  former.  There  is  a  strong 
paralleUsm  between  the  two  curves,  but  the  one  for  the  humid  season 
is  slightly  flatter  than  the  one  for  the  arid  season.  This  means  that, 
so  far  as  concerns  the  evaporation  conditions  alone,  there  is  a  less 
differentiation  between  Desert  and  Forest  in  the  summer  rainy  season 
than  there  is  in  the  arid  portion  of  the  summer.  The  pronounced  drop 
in  evaporation  from  7,000  to  8,000  feet  is  particularly  significant,  as 
the  former  elevation  marks  the  lower  edge  of  the  Forest,  while  the 
latter  is  in  the  midst  of  the  best  stands  of  pine.  It  is  possible  that  the 
forest  itself  interferes  with  air  movements  near  the  ground  in  such  a 
way  as  to  be  responsible  for  the  sharp  fall  in  evaporation. 

In  order  to  exhibit  the  seasonal  march  of  evaporation  rate  at  the 
several  altitudes  the  curves  of  figure  11  have  been  drawn.     These 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


65 


curves  show  the  averages  between  the  south  and  north  slopes  for  each 
station  at  each  reading,  being  expressed  in  cubic  centimeters  of  evapo- 
ration per  day.    Here  again  is  brought  out  the  pronounced  fall  in 


no 

- 

1 

1 

1                          1 

1                          1                          \-              -1 

100 

- 

\ 

- 

90 

. 

\ 

80 

- 

.^r^ 

^^^^x 

.^ 

. 

70 

-. 

'^     o/ 

^~~^^ 

\\\ 

_ 

^/    ^^\ 

vS 

60 

- 

y^^^ 

^o 

\\\ 

y "^^""""^--^ 

50 

- 

•^-A  X 

yf^'"'    ^^^^^^\^ 

^""^^^^^ 

V/^''          ^ _______^        ^\ 

40 

- 

\x 

l/<^^^^^^^c:i— ^TX 

30 

- 

""-t^oo               \ 

^:^y^^^^        ^"^  " 

20 

- 

V. 

y"^ 

10 

- 

\..'''           ^^-^,-""^-~.^      . 

0 

, 

, 

,                              1 

, 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

Fia.  11. — Graphs  showing  seasonal  march  of  evaporation  rate  at  6  altitudes  in  the  Santa  Cata- 
linas  in  1911.  Amounts  are  average  daily  losses  from  the  atmometer,  and  each  reading  is 
the  average  of  one  on  a  north  slope  and  one  on  a  south  slope. 


rate  which  follows  the  advent  of  the  summer  rains  and  the  cloudy 

and  relatively  humid  weather  by  which  they  are  accompanied.    After 

the  period  of  heavy  rains  by  which  the 

humid  mid-summer  was  opened  in  the  last 

days  of  June  and  early  days  of  July  there 

was  a  sUght  rise  in  evaporation,  followed 

by  a  slight  fall  in  late  August  and  early 

September.    The  curves  for  3,000  and 

4,000  feet  accompany  each  other  closely 

after  the  first  two  readings,  and  the  curves 

for  6,000  and  7,000  feet  accompany  each 

other  closely  throughout   the  summer. 

The  curve  for  8,000  feet  stands  always 

well  apart  from  that  for  7,000  feet.    The 

grouping  of  these  curves  is,  therefore, 

analogous  to  the  natural  subdivisions  of 

the  vegetation.    The  readings  taken  in  the 

Desert  region  at  3,000  and  4,000  feet,  those  taken  in  the  Encinal  and  the 

lower  edge  of  the  Forest  at  5,000, 6,000,  and  7,000  feet,  and  the  one  series 


Fig.  12. — Graphs  showing  altitudinal 
decrease  in  rate  of  evaporation  in 
the  Santa  Catalinas  in  arid  fore- 
summer  (heavy  line)  and  humid 
mid-summer  (light  line)  of  1911. 


66 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


taken  in  the  heart  of  the  Forest  stand  apart  in  three  loosely  defined 
groups  in  close  parallelism  to  the  zonation  of  the  vegetation  itself. 

During  the  arid  fore-summer  the  evaporation  at  5,000  feet  is  similar 
to  that  at  4,000  feet,  while  during  the  humid  mid-summer  it  is  more 
nearly  like  that  of  the  6,000-foot  station.  In  other  words,  the  advent 
of  the  rains  causes  the  evaporation  conditions  of  the  Upper  Encinal 
and  lower  Forest  region  to  extend  downward  into  the  Lower  Encinal. 

The  significance  of  slope  exposure  in  determining  evaporation  rate 
is  indicated  in  figures  13  and  14.  In  these  graphs  the  vertical  gradients 
of  evaporation  at  the  six  elevations  are  shown  separately  for  the  instru- 
ments on  the  south  slopes  and  the  north  slopes  at  each  station.  The 
gradients  for  the  arid  fore-summer  and  for  the  humid  mid-summer 
are  shown,  as  well  as  the  curves  for  the  entire  summer.  In  the  arid 
season  there  is  even  a  slightly  greater  evaporation  on  north  slopes  at 
4,000  and  5,000  feet  than  there  is  on  south  slopes,  but  this  condition 


Fig.  13. — Graphs  showing  altitudinal  decrease  in  rate  of  evaporation  in  the  Santa  Catalinas  on 

south-facing  slopes  (heavy  line)  and  on  north-facing  slopes  (light  line)  in  arid  fore-summer 

of  1911. 
Fig.  14. — Graphs  showing  altitudinal  decrease  in  rate  of  evaporation  in  the  Santa  Catalinas  on 

south-facing  slopes  (heavy  line)  and  on  north-facing  slopes  (light  hne)  in  humid  mid-summer 

of  1911. 

is  reversed  at  the  higher  elevations.  In  the  humid  season  there  is  also 
a  slightly  greater  rate  of  evaporation  on  the  north  slope  at  4,000  feet, 
while  all  of  the  higher  stations  show  an  almost  uniformly  greater  rate 
on  the  south  slopes.  It  is  impossible  to  explain  the  cases  in  which  the 
evaporation  was  greater  on  north  slopes  than  on  south  ones.  It  is 
possible,  of  course,  that  they  require  no  explanation  but  are  typical  of 
the  extremely  arid  conditions  of  the  lowest  elevations  at  the  driest 
time  of  the  year.  They  are  at  least  accordant  with  the  fact  that  the 
soil  moisture  is  sometimes  greater  on  the  south  slopes. 

The  summer  averages  show  a  difference  of  from  5  to  10  c.c.  per  day 
between  the  evaporation  on  opposed  slopes,  in  readings  of  35  to  45  c.c. 
per  day  or  less.  As  the  actual  amounts  of  evaporation  fall  with  increas- 
ing altitude,  the  difference  between  the  opposed  slopes  becomes  pro- 
portionately greater. 

As  in  the  case  of  all  climatological  data,  it  would  be  impossible  to 
state  the  normal  conditions  of  evaporation  at  the  various  altitudes 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  67 

and  on  opposed  slopes  without  instrumental  data  for  several  series  of 
stations  and  covering  several  years.  It  is  possible,  nevertheless,  to 
state  from  the  data  presented  that  (a)  the  rate  of  evaporation  through 
the  arid  and  humid  summer  seasons  is  about  33^  times  as  great  on  the 
desert  as  it  is  at  8,000  feet;  (6)  the  rates  of  evaporation  are  approxi- 
mately half  as  great  in  the  humid  mid-summer  as  they  are  in  the  arid 
fore-summer;  (c)  at  the  middle  and  higher  altitudes  the  evaporation 
on  north  slopes  is  less  than  on  south  slopes ;  (d)  the  difference  between 
the  amounts  of  evaporation  on  north  and  south  slopes  becomes  greater 
with  increase  of  altitude,  in  proportion  to  the  amounts  of  each. 

HUMIDITY. 

The  prevalence  of  low  atmospheric  humidities  is  one  of  the  most 
pronounced  features  of  desert  climate  and  is  an  extremely  potent  factor 
in  causing  the  high  rates  of  evaporation  that  have  been  shown  to  occur 
at  the  lowest  stations  in  the  Santa  Catalina  Mountains.  The  relative 
humidity  is  lowest  in  the  arid  fore-summer,  although  it  is  sometimes 
nearly  as  low  for  brief  periods  in  the  arid  after-summer.  During  the 
two  rainy  seasons  the  humidity  is  extremely  variable  and  may  fluctuate 
through  a  daily  range  of  as  much  as  70  per  cent.  The  daily  curve  of 
humidity  is  extremely  uniform  during  the  cloudless  days  of  April,  May, 
and  June,  falling  rapidly  during  the  early  forenoon  to  mid-day  values 
as  low  as  5  and  10  per  cent,  and  rising  slowly  through  the  late  afternoon 
and  more  rapidly  during  the  night  to  a  daily  maximum  of  20  to  30  per 
cent  just  before  sunrise.  Cloudy  days  in  the  arid  seasons  cause  a  higher 
minimum  but  seldom  raise  the  maximum  above  40  per  cent  unless 
there  is  a  trace  of  rainfall. 

The  humidities  of  the  mountain,  varying  with  altitude  and  with  the 
seasons,  possess  their  greatest  importance  for  vegetation  in  their  role 
as  joint  determinants  of  the  rate  of  evaporation.  The  altitudinal 
gradient  of  humidity  has,  therefore,  been  most  satisfactorily  investi- 
gated when  it  has  been  measured  together  with  temperature  and  wind 
in  the  collective  effect  of  these  climatic  factors  upon  the  evaporative 
power  of  the  air.  It  is  not  without  interest,  nevertheless,  to  know 
something  of  the  relative  humidities  which  are  prevalent  at  the  moun- 
tain altitudes  and  are  partially  responsible  for  the  rates  of  evaporation 
encountered  there. 

In  spite  of  the  pronounced  altitudinal  changes  of  vegetation  and  of 
climatic  conditions  which  have  been  discussed  (or  are  yet  to  be  treated), 
there  are  so  many  features  of  the  Encinal  and  Forest  vegetation  that 
strongly  suggest  the  Desert  (see  p.  36)  that  it  seemed  particularly 
desirable  to  secure  readings  of  relative  humidity  at  the  forested  alti- 
tudes in  the  arid  fore-sunamer.  The  few  figures  to  be  given  here  were 
secured  with  a  sHng  psychrometer  and  converted  to  percentages  by 
the  use  of  Marvin's  tables. 


68  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

On  May  22,  1911,  in  the  east  fork  of  Sabino  Basin,  at  3,400  feet 
elevation,  the  humidity  at  3^  30°^  p.  m.  was  6  per  cent,  at  6  p.  m.  it 
was  8  per  cent,  and  at  8^  30°^  p.  m.  it  was  12  per  cent.  At  4^  30"^  a.  m., 
on  the  following  day,  the  humidity  had  risen  to  24  per  cent.  These 
figures  show  the  prevalence  of  desert  humidities  at  a  locality  which  is 
low  in  elevation  but  is  well  in  the  heart  of  the  mountain  mass  as  a  whole. 
At  Marshall  Gulch,  at  7,600  feet  in  the  Forest  region,  on  May  20, 1911, 
the  humidity  at  11^  30°"  a.  m.  was  10  per  cent,  and  it  was  the  same  at 
4^  30°"  p.  m.  At  O**  15°"  a.  m.  on  the  following  day  the  humidity  was 
16  per  cent,  and  at  3  p.  m.  it  was  11  per  cent.  Although  these  figures 
are  roughly  twice  those  of  the  readings  in  Sabino  Basin,  they  are  never- 
theless indicative  of  a  low  humidity  for  a  forested  locality  and  show 
that  in  the  arid  fore-summer  there  are  days  on  which  the  humidity  is 
nearly  as  low  as  it  is  on  the  Desert. 

A  number  of  humidity  readings  were  taken  in  June  1911,  but  none 
of  them  showed  as  low  values  as  those  just  mentioned.  In  Bear  Canon, 
at  6,100  feet  elevation,  on  June  21  (a  dull  and  intermittently  cloudy 
day),  the  humidity  was  46  per  cent  at  1  p.  m.  and  42  per  cent  at  3  p.  m., 
falling  to  41  per  cent  at  7  p.  m.  In  Marshall  Gulch  on  June  23  (a  nearly 
cloudless  day),  the  humidity  at  5  a.  m.  was  33  per  cent  and  it  fell 
steadily  to  22  per  cent  at  12  noon,  with  a  temporary  rise  during  an 
interval  of  cloudiness  at  10  a.  m.  In  the  afternoon  the  percentages 
rose  from  25  per  cent  at  3  p.  m.  to  29  per  cent  at  6  p.  m.,  but  fell  again 
to  26  per  cent  at  8  p.  m.  The  highest  humidity  observed  at  Marshall 
Gulch  was  48  per  cent  at  4^  30™  p.  m.  on  June  8,  1911,  after  the  summit 
of  the  mountain  had  been  covered  several  hours  with  cumulus  clouds. 

Continuous  records  of  relative  humidity  have  been  secured  in  yellow 
pine  forest  at  the  Fort  Valley  Experiment  Station  at  7,200  feet  eleva- 
tion, near  Flagstaff,  Arizona,  by  Pearson.*  The  monthly  mean  values 
for  May  and  June  (1909-1912)  are  38  and  34.9  per  cent  respectively. 
Some  of  the  lowest  extremes  involved  in  the  composition  of  these  means 
have  been  kindly  conamunicated  to  the  writer  by  Pearson.  The  number 
of  days  in  June  on  which  the  humidity  fell  to  16  per  cent  or  less  was 
as  follows:  8  days  in  1909;  11  days  in  1910;  6  days  in  1911.  The  lowest 
humidities  were  a  single  occurrence  of  10  per  cent  and  two  occurrences 
of  11  per  cent.  Humidities  as  low  as  11  per  cent  also  occur  in  July, 
and  values  as  low  as  16  per  cent  occur  between  May  and  October. 

These  figures  for  the  Coconino  Plateau  are  in  agreement  with  the 
lowest  figures  secured  at  Marshall  Gulch  in  May  1911,  and  show  that 
desert  humidities  are  of  frequent  occurrence  in  the  arid  fore-summer, 
both  on  isolated  mountains,  such  as  the  Santa  Catalinas,  and  on  ex- 
tended plateaus  in  the  midst  of  the  arid  region.  On  the  days  that 
exhibit  such  low  humidities  at  higher  elevations  there  is  practically  a 

*  Pearson,  G.  A.  A  Meteorological  Study  of  Parks  and  Timbered  Areas  in  the  Western 
Yellow  Pine  Forests  of  Arizona  and  New  Mexico.    Mo.  Weather  Rev.,  41:  1615-1629,  1914. 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  69 

flat  altitudinal  gradient  of  humidity.  The  difference  between  the 
observed  humidities  of  6  per  cent  in  Sabino  Basin  and  12  per  cent  at 
Marshall  Gulch  is  a  very  small  one,  and  would  doubtless  register  very 
small  differences  on  the  rate  of  evaporation  under  otherwise  identical 
conditions.  The  differences  in  evaporation  actually  found  to  exist 
between  the  base  and  summit  of  the  mountain  are  to  be  ascribed  to  the 
nocturnal  humidities,  which  are  higher  in  the  Forest  than  on  the  Desert, 
to  the  greater  frequency  of  cloudiness  at  higher  elevations  in  the  arid 
fore-summer,  to  the  lower  temperatures  at  higher  altitudes  (especially 
at  night),  and  to  the  wind  protection  afforded  by  the  forest  cover  itself. 

TEMPERATURE. 

The  investigation  of  temperature  on  the  Santa  Catalinas  has  been 
carried  on  with  a  view  to  determining  the  decrease  in  length  of  the 
frostless  season  which  accompanies  increase  of  altitude,  the  normal 
decrease  of  temperature  with  increasing  altitude,  and  the  departures 
from  the  normal  gradient  of  decrease  which  are  due  to  the  nature  of 
the  topographic  relief  and  to  other  causes.  The  results  secured  afford 
an  outline  of  the  major  temperature  features  which  are  capable  of 
influencing  the  distribution  or  seasonal  activities  of  the  plants  of  the 
Desert,  Encinal,  and  Forest  regions. 

The  character  of  the  temperature  conditions,  and  their  relation  to 
altitude  and  topography,  in  an  isolated  desert  mountain  is  not  without 
complexities  which  make  it  impossible  to  predict  the  conditions  for 
vegetation  in  a  given  locaHty  through  a  knowledge  of  its  altitude 
and  of  general  meteorological  theory.  The  relative  smallness  of  the 
entire  mountain  mass  and  its  position  in  the  midst  of  arid  plains  make 
its  temperature  conditions  very  different  from  those  of  extensive 
plateaus  of  the  same  elevation.  The  currents  of  wann  air  which  ascend 
by  day  and  the  streams  of  cold  air  which  descend  by  night  serve  to 
increase  the  diurnal  amplitude  of  temperature  in  certain  situations  and 
to  give  striking  differences  within  very  short  distances.  Differences 
of  slope  exposure  bring  about  differences  of  diurnal  warming  and 
nocturnal  cooling  of  the  soil,  and  these  differences  affect  the  general 
temperature  conditions  and  also  directly  influence  the  vegetation. 
The  differences  of  diurnal  warming  and  nocturnal  cooling  which  exist 
between  the  relatively  bare  soils  of  the  Desert  and  Encinal  regions 
and  those  of  the  Forest,  with  their  heavy  cover  of  vegetation,  their 
litter  of  leaves  and  high  humus  content,  are  also  considerable  and  tend 
to  lessen  the  importance  of  topography  at  the  higher  elevations. 

Temperature  readings  have  been  secured  at  two  localities,  at  ele- 
vations of  5,300  and  7,600  feet,  respectively,  since  the  early  summer  of 
1908.  Since  1911  a  series  of  thermometers  has  been  exposed  at  the 
rainfall  situations  at  4,000,  6,000,  and  7,000  feet,  and  during  1913  and 
1914  a  complete  series  of  thermometers  was  maintained  at  1,000-foofc 


70  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

intervals  from  4,000  to  9,000  feet.  All  of  the  instruments  in  this  series 
were  located  on  the  summits  of  ridges,  so  as  to  give  comparable  readings 
from  similar  topographic  situations.  In  addition  to  the  six  instruments 
in  this  series  there  were  also  thermometers  in  the  bottoms  of  cafions  at 
5,000,  6,000,  and  7,600  feet;  a  thermometer  was  exposed  on  the  top 
of  the  fire  tower  of  the  Forest  Service  on  Mount  Lemmon,  the  actual 
elevation  of  the  instrument  being  9,225  feet,  and  thermometers  were 
buried  in  the  topmost  layer  of  soil  at  6,000  and  8,000  feet. 

Alcoholic  minimum  thermometers  were  used  in  the  earlier  years  of 
these  observations,  but  were  replaced  by  mercuric  Six's  thermometers 
in  1913  and  1914.  Various  types  of  thermometer  have  been  used  at 
the  station  at  7,600  feet,  and  as  many  as  three  instruments  have  been 
exposed  simultaneously  at  that  place.  All  thermometers  have  been 
calibrated  before  use  and  have  been  verified  in  place  from  time  to  time 
by  comparison  with  a  portable  thermometer  of  known  error.  The 
readings  of  the  thermometers  have  been  taken  at  irregular  intervals, 
as  opportunity  afforded,  and  most  of  the  figures  secured  are  for  periods 
of  several  weeks,  or  for  the  several  months  which  elapse  between  the 
last  visit  in  the  autumn  and  the  first  in  the  spring.  Only  at  the  7,600- 
and  9,000-foot  stations  has  it  been  possible  to  expose  the  thermometers 
in  such  a  manner  as  to  secure  reliable  maxima;  at  all  other  stations 
the  only  data  secured  have  been  the  absolute  minima  for  the  intervals 
between  visits.  The  placing  of  the  thermometers  in  small  boxes,  with 
numerous  perforations,  has  made  possible  the  securing  of  good  minima, 
but  no  record  has  been  made  of  the  maxima  secured  under  such  con- 
ditions of  exposure.  The  conspicuousness  of  adequate  instrument 
shelters  would  have  invited  human  interference  with  the  thermometers 
which  would  have  been  productive  of  errors. 

A  few  records  of  temperature  from  the  same  locality  for  a  number 
of  consecutive  days  have  been  secured  by  Professor  J.  G.  Brown  and  by 
Dr.  H.  A.  Spoehr,  as  well  as  by  the  writer.  A  large  number  of  single 
observations  of  minima  and  of  current  temperatures  have  been  made 
by  the  writer  at  various  localities,  and  it  has  been  possible  to  use  these 
in  connection  with  data  from  the  regular  stations  in  determining  the 
normal  gradient  of  temperature  decrease  and  in  ascertaining  the  verti- 
cal shortening  of  the  frostless  season. 

LENGTH  OF  FROSTLESS  SEASON. 

It  has  been  impossible,  for  the  most  part,  to  make  direct  observations 
of  the  dates  of  last  vernal  and  first  autumnal  occurrence  of  a  tempera- 
ture of  32°  F.  at  the  several  stations  on  the  Santa  Catalinas.  The 
dates  at  which  visits  were  made  to  the  mountain  were  occasionally 
such  as  to  estabhsh  the  dates  exactly  for  one  of  the  stations,  and  in 
several  cases  visits  were  made  at  such  frequent  intervals  as  to  place 
the  date  within  a  week  or  two.    In  the  majority  of  cases,  however, 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


71 


the  frost  dates  which  limited  the  growing  season  fell  between  the  last 
visit  of  autumn  and  the  first  one  of  spring.  This  has  particularly  been 
the  case  with  all  of  the  lower  stations.  These  circumstances  have 
made  it  necessary  to  resort  to  an  indirect  method  of  determining  the 
dates,  which  is  as  follows:  A  series  of  graphs  was  drawn  showing  the 
march  of  the  weekly  absolute  minimum  temperatures  at  the  Desert 
Laboratory,  as  registered  by  thermograph,  for  the  years  covered  by 
the  mountain  records.  Each  reading  of  minimum  temperature  for  a 
given  station  was  then  compared  with  the  minimum  for  the  same 
period  at  the  Desert  Laboratory,  and  the  total  number  of  such  differ- 
ences was  averaged.  In  this  manner  it  was  possible  to  secure  the 
figures  given  in  tables  10  and  11. 

Table  10. — Allitudinal  shortening  of  the  frostless  season  in  the  Santa  Catalina  Mountains, 
as  shown  by  the  dates  of  the  last  spring  occurrence  and  the  first  autumn  occurrence  of  a 
temperature  of  32°  at  3  altitudes  in  1908,  1909,  and  1910. 


Station,  etc. 

Year. 

Last  32° 
in  spring. 

First  32° 
in  autumn. 

Desert  Laboratory;     elevation,   2,663 
feet;    length  of  frostless  season,  285 
days. 

fl908 
U909 
il910 

Feb.  20 
Mar.  15 
Feb.  25 

Nov.  30 
Nov.  30 
Dec.  31 

Mar.    1 

Dec.  10 

Xero-Montane    Garden;        elevation, 
5,300  feet;  length  of  frostless  season, 
195  days. 

[1908 
]l909 
[1910 

May  10 
Apr.  12 
Apr.  20 

Oct.   10 
Nov.  10 
Nov.  26 

Apr.  24 

Nov.    5 

Montane    Garden,     Marshall    Gulch; 
elevation,  7,600  feet;  length  of  frost- 
less season,  126  days. 

Average  dates                               

fl908 
U909 
[1910 

June  15 
May  31 
May    9 

Sept.  25 
Sept.  26 
Oct.   17(?) 

May  29 

Oct.     2 

With  a  knowledge  of  the  average  difference  between  the  minimum 
temperature  at  the  Desert  Laboratory  and  at  a  given  station,  and  with 
the  graph  showing  the  march  of  minima  at  the  Laboratory,  it  was 
possible  to  locate  the  approximate  date  of  the  last  and  first  occurrence 
of  32°  at  the  mountain  station.  Such  a  graph  for  1911  is  given  in 
figure  17,  together  with  the  graph  of  march  of  minimum  temperatures 
at  the  Montane  Plantation  in  Marshall  Gulch,  at  7,600  feet.  It  will 
be  noted  that  the  graph  for  the  Laboratory  rises  by  several  pronounced 
jumps  during  March,  April,  and  May,  and  falls  by  precipitate  stages 
during  September,  October,  and  November.  The  relatively  sudden 
advent  of  summer  and  of  winter  is  an  invariable  annual  occurrence, 
and  it  has  helped  to  make  more  accurate  the  estimation  of  the  limiting 
dates  for  the  mountain  stations.    In  the  cases  in  which  a  minimum 


72 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


temperature  of  32°  or  less  was  registered  at  a  station  during  an  interval 
of  two  or  three  weeks,  the  date  of  such  minimum  could  be  determined 
by  finding  the  exact  date  of  the  lowest  temperature  for  the  same  period 
at  the  Desert  Laboratory,  and  such  determinations  undoubtedly  have 
a  very  slight  possibility  of  error.  Taking  into  account  the  number  of 
direct  observations  and  the  larger  number  of  estimations,  the  limiting 
dates  of  the  frostless  season,  given  in  tables  10  and  11,  may  contain 
errors  of  as  much  as  7  to  10  days.  The  swamping  of  these  errors  by 
averaging  the  dates  for  the  6  years  of  observation  reduces  the  probable 
error  to  about  5  days. 

Table  11. — The  altitudinal  shortening  of  the  frostless  season  in  the  Santa  Catalina  Mountains, 
as  shown  by  the  dates  of  the  last  spring  occurrence  and  the  first  autumn  occurrence  of  a 
temperature  of  32°  at  5  altitudes  in  1911,  1912,  and  1913. 

[Data  for  9,000  feet  are  partially  interpolated.] 


Station. 

Year. 

Last  32° 
in  spring. 

First  32° 
in  autumn. 

Desert  Laboratory;     elevation,  2,663 
feet;    length  of  frostless  season,  282 
days. 

i 

1911 
1912 
1913 

Feb.  20 
Feb.  26 
Mar.  31 

Dec.  25 
Dec.    9 
Dec.     8 

Mar.    7 

Dec.  14 

Climatological  Station;  elevation,  5,000 
feet;    length  of  frostless  season,  248 
days. 

i 

ri9ii 

1912 
1913 

Feb.  27 
Apr.  15 
Apr.  14 

Dec.     4 
Dec.    9 
Nov.  21 

Mar.  30 

Dec.     1 

Climatological  Station;  elevation,  7,000 
feet;    length  of  frostless  season,  187 
days. 

fl911 
■^1912 
[1913 

Apr.     3 
May  13 
May    5 

Oct.   30 

Nov.  18 
Oct.    13 

Apr.  27 

Oct.  31 

Marshall  Gulch;    elevation,  7,600  feet; 
length  of  frostless  season,  148  days. 

Average  dates 

1 

1911 
1912 
1913 

May  16 

May  18 
May    9 

Oct.  30 
Oct.     2 
Sept.  26 

May  14 

Oct.  9 

Mount  Lemmon;  elevation,  9,000  feet; 
length  of  frostless  season,  122  days. 

fl911 
J1912 
[1913 

May  29 
May  20 
June  16 

Oct.   16 
Oct.     7 
Sept.    8 

June    1 

Sept.  30 

In  figure  2  are  shown  separately  the  curves  for  altitudinal  shortening 
of  the  frostless  season  for  the  years  1908,  1909,  and  1910,  and  the  years 
1911,  1912,  and  1913.  In  the  latter  group  of  years  the  advent  of  spring 
was  nearly  a  month  earlier  at  the  middle  altitudes  than  it  was  during 
the  former  years,  and  the  advent  of  winter  was  correspondingly  later, 
in  spite  of  the  fact  that  the  frostless  season  was  of  approximately  the 
same  length  at  the  Desert  Laboratory  during  the  two  periods.    A  con- 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


73 


sideration  of  the  two  sets  of  curves  is  more  fruitful  than  the  possession 
of  their  average,  as  it  shows  the  extent  of  a  fluctuation  which  must 
be  a  common  and  normal  feature  of  the  climatology  of  the  mountain, 
just  as  it  is  of  every  locality,  regardless  of  its  topographic  location. 

The  average  length  of  the  frostless  season  at  the  Desert  Laboratory 
is  about  38  weeks,  at  5,000  feet  in  the  Santa  CataUnas  it  is  about 
30  weeks,  while  at  8,000  feet  it  is  about  17  weeks.  The  period  of  safe 
plant  activity  is,  therefore,  less  than  half  as  long  in  the  Forest  region 
of  the  mountain  as  it  is  in  the  Desert  of  the  Santa  Cruz  Valley.  The 
altitudinal  abbreviation  of  the  frostless  season  is  of  primary  importance 
to  vegetation,  especially  as  it  is  accompanied  by  a  series  of  inseparable 
features  of  temperature,  such  as  the  lower  range  of  the  entire  daily 
curve  of  temperature,  the  attainment  of  lower  minima,  the  more 


uuo 

/ 

MARSHALL 

GULCH 

\ 

./ 

chlarson' 

j  MILL 

^ 

coo 

■    ■/ 

=A 

/ 

/ 

FL- 

GSTAFF 

\ 

\ 

/_ 

FT   HUACHUC 

, 

7^ 

OCHISE 

000 

/ 

BtNSON 

\ 

,000 

/ 

"■ 

/ 

TUCSON 

\ 

,000 

PHOENIX 

VUMA 

0 

JAN. 

MAR. 

APR.                      MAY 

JUNE 

JUUV 

AUG. 

SEPT. 

OCT, 

NOV. 

DEC. 

Fig.  15. — Schematic  representation  of  length  of  frostless  season  at  different  altitudes  in  Arizona, 
together  with  curves  for  limits  of  growing  season  at  successive  altitudes  in  the  Santa  Cata- 
Unas for  1909  to  1914  inclusive. 

frequent  occurrence,  and  the  longer  duration  of  freezing  temperatures. 
One  of  the  cardinal  features  of  importance  in  the  altitudinal  shortening 
of  the  growing  season  is  the  concomitant  shortening  of  the  arid  fore- 
summer.  The  rising  temperatures  of  spring  call  the  vegetation  of  the 
Desert  into  activity  at  a  time  of  the  year  when  extremely  arid  condi- 
tions are  bound  to  prevail  for  from  14  to  16  weeks.  At  the  summit  ot 
the  mountain,  however,  the  advent  of  spring  is  only  5  or  6  weeks  in 
advance  of  the  earliest  of  the  mid-summer  rains.  In  other  words,  the 
most  trying  season  of  the  year  is  shortened  on  the  mountain  by  the 
inhibitory  effects  of  low  temperatures,  so  that  the  arid  fore-summer  is 
only  one-third  as  long  in  the  Forest  as  in  the  Desert.  These  relations 
are  graphically  represented  in  figure  2,  which  shows  both  the  curves 
of  the  frost  dates  and  the  incidence  of  the  rainy  seasons.  More  will 
be  said  in  regard  to  this  subject  in  a  succeeding  section  (see  p.  93). 


74 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


In  order  to  compare  the  altitudinal  shortening  of  the  frostless  season 
on  the  Santa  Catahnas  with  the  same  datum  for  a  series  of  valley 
stations  located  at  progressive  altitudes,  figures  were  collected  which 
are  shown  in  table  12.  These  figures  are  based  on  the  last  vernal  and 
first  autumnal  occurrence  of  a  temperature  of  32°,  for  1903  to  1912 
inclusive,  without  regard  to  the  reports  of  frost  made  by  the  voluntary 
observers  at  these  stations.  In  figure  15  the  length  of  the  frostless 
season  at  the  several  stations  is  graphically  shown  by  horizontal  lines, 
and  the  limits  of  the  frostless  season  for  the  Santa  Catalinas  (for  1909 
to  1914)  are  shown  by  oblique  fines. 

Spring  opens  at  an  earlier  date  at  3,000  and  4,000  feet  on  the  Santa 
Catalinas  than  it  does  at  Tucson  and  Benson,  but  at  4,000  and  5,000 
feet  it  does  not  open  at  so  early  a  date  as  it  does  at  Cochise  and  Fort 
Huachuca.    At  all  four  of  the  elevations  mentioned  the  close  of  the 

Table  12. — The  altitudinal  shortening  of  the  frostless  season  in  southeastern  Arizona,  as 
shown  by  the  dates  of  the  last  spring  occurrence  and  the  first  autumn  occurrence  of  a  tempera- 
ture of  33°  at  eight  stations  at  graduated  altitudes,  in  the  decade  of  1903  to  1912. 


station. 

Last  32° 
in  spring. 

First  32° 
in  autumn. 

Length  of 

frostless 

season  (days). 

Yuma  141  feet  . 

Jan.   28 
Feb.     1 
Mar.  15 
Mar.  26 
Mar.  10 
Mar.  26 
June  11 
May  13  t 

Dec.  18 
Dec.  18 
Nov.  19 
Nov.    7 
Nov.    6 
Nov.    8 
Sept.  24 
Oct.   19  t 

325 
321 
248 
226 
241 
227 
105 
159 

Phoenix,  1,108  feet 

Tucson,  2, .390  feet 

Cochise,  4,219  feet 

Fort  Huachuca,  5,100  feet 

Flagstaff,  6,907  feet 

Chlarson's  Mill,  7,200  feet  *.  .  . . 

*  The  elevation  of  Chlarson's  Mill  is  reputed  by  the  proprietor  to  be  8,000  feet,  and  it  is  so 
stated  in  the  publications  of  the  Weather  Bureau.  Several  aneroid  determinations  by  the  writer 
indicate  that  it  is  approximately  7,200  feet. 

t  These  dates  are  based  on  an  incomplete  record. 

growing  season  comes  sooner  at  the  valley  stations  than  it  does  on  the 
mountains.  The  length  of  the  frostless  season  at  Flagstaff  is  notably 
shorter  than  it  is  at  the  same  elevation  in  the  mountains. 

The  advent  of  spring  is  retarded  at  Tucson  and  Benson  by  the  cold- 
air  drainage  of  the  Santa  Cruz  and  San  Pedro  rivers  respectively. 
Cochise  is  situated  in  the  middle  of  the  eastern  bajada  of  the  Dragoon 
Mountains  and  Fort  Huachuca  at  the  top  of  the  northern  bajada  of 
the  Huachuca  Mountains.  Each  of  these  stations  is  therefore  removed 
from  the  operation  of  cold-air  drainage,  as  is  manifested  by  the  failure 
of  their  greater  altitude  to  impose  upon  them  shorter  frostless  seasons 
than  those  of  Tucson  and  Benson. 

The  length  of  the  frostless  season  at  Marshall  Gulch  and  at  the 
similarly  situated  mountain  station  at  Chlarson's  Mill  (Pinaleno  Moun- 
tains) is  greater  than  at  Flagstaff,  which  is  at  a  slightly  lower  altitude. 
This  is  to  be  attributed  partly  to  the  higher  latitude  of  Flagstaff,  but 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


75 


chiefly  to  its  location  on  an  extensive  elevated  plateau  and  to  its 
proximity  to  the  cold-air  flow  from  the  San  Francisco  Peaks  and  other 
neighboring  elevations. 

It  may  be  said,  in  general,  that  the  frostless  season  is  longer  on  the 
ridges  of  an  isolated  mountain  than  it  is  in  adjacent  valleys  at  the 
same  elevations.  Although  the  advent  of  spring  at  Cochise  and  Fort 
Huachuca  is  earlier  than  on  the  mountains,  the  arrival  of  autumn  is 
earlier  also,  so  that  these  two  stations  show  an  equality  in  length  of 
frostless  season  with  the  mountain  ridges  without  a  correspondence 
with  them  in  the  dates  of  commencement  and  close. 

NORMAL  ALTITUDINAL  TEMPERATURE  GRADIENT. 

The  temperatures  which  have  been  secured  on  the  Santa  Catalinas 
do  not  form  an  altogether  satisfactory  basis  for  the  determination  of 

Table  13. — Average  differences  between  all  observed  minimum  temperatures  at  stations  situ- 
ated on  ridges  in  the  Santa  Catalina  Mountains  and  the  minima  for  the  same  days  or 
periods  at  the  Desert  Laboratory. 


Station. 

1911. 

1912. 

1913. 

1914. 

Average. 

4,000  feet... 
5  000  feet  . 

+7.2 

+    .1 

1.6 

8.1 

7.6 

14.0 

20.1 

20.8 

+  1.9 

8.1 

9.2 

13.7 

20.1 

25.9 

6,000  feet... 
7,000  feet... 
8  000  feet  . 

12.5 
.6.1 



7.5 
11.1 

9,000  feet . . . 

31.1 

[The  plus  sign  indicates  a  higher  temperature  at  the  mountain  station.] 

the  normal  altitudinal  gradient,  a  datum  which  should  be  derived  from 
extended  series  of  mean  temperatures.  However,  in  the  absence  of  an 
ideal  collection  of  records  from  the  several  altitudes  these  imperfect 


Fia.  16. — Graph  showing  altitudinal  fall  in  temperature  in  the  Santa  Catalina  Mountains 
(A),  and  lines  showing  rate  of  fall  in  free  air  (E)  on  Pike's  Peak  (C),  on  the  Sierra 
Nevada  (D),  and  average  rate  for  17  extra-tropical  mountains  (B). 

data  have  been  used  as  the  basis  of  an  approximate  determination  of 
the  gradient  of  fall  of  temperature  with  increase  of  altitude.     The 


76 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


minimum  temperatures  at  the  several  stations  are  the  only  ones  that 
have  been  used,  and  they  have  been  in  all  cases  compared  wdth  the 
minimum  for  the  same  period  at  the  Desert  Laboratory.  The  average 
differences  between  the  minima  of  the  mountain  stations  and  those  of 
the  Laboratorj'^  are  shown  in  table  13,  and  it  is  these  differences  that 


Table  14. — Daily  maximum  and  minimum 
at  the  Desert  Laboratory  for 

temperatures  at  Marshall  Gulch  {7,600  feet)  and 
June,  July,  and  August  1911. 

Day 

of 
month. 

June. 

July. 

August. 

Minima. 

Maxima. 

Mim'ma. 

Maxima. 

Minima. 

Maxima. 

M.G. 

D.  L. 

M.G. 

D.  L. 

M.G. 

D.  L. 

M.G. 

D.  L. 

M.G. 

D.  L. 

M.G. 

D.  L. 

1st 

65 
65 
69 
73 

78 

80 

"74" 
64 
70 
67 

67 

81 
83 
95 
100 
103 

98 

51 
62 

48 
46 
48 

76 
81 

77 
76 
74 

81 
77 
80 
83 
81 

loe" 

103 

103 

"91" 
94 

89 
102 
105 
107 
110 

109 
107 
105 
107 
100 

88 
93 
94 

2d 

48 
46 

3d 

4th. .    .  . 

5th.      .  . 

6th 

46 

7th 

48 
60 
48 
50 

48 
50 
50 
49 
54 

62 
62 
49 
50 
49 

52 
51 

46 

48 

80 
80 
74 
76 

77 
79 
78 
76 
81 

82 
73 
73 
79 
71 

68 
69 
67 

74 

84 
74 
78 
74 

73 
70 
75 

76 
81 

74 
71 
74 
76 
75 

63 
61 
62 

8th 

9th 

10th 

nth 

12th 

13th 

14th 

15th 

16th 

17th 

48 
48 
44 

46 
48 
49 
41 
41 

41 

75 
76 
73 

76 
73 

77 
74 
74 

70 
69 
71 
81 
73 

76 

78 
73 
76 
73 

73 
75 

78 
78 
74 

77 

"79" 

67 
66 
59 
70 

74 

76 

"79" 

74 
72 

81 
81 
83 
83 

85 

85 
84 
84 
76 

106 
103 
104 

97 
96 
85 
98 
97 

98 
102 
106 
101 

99 

106 
104 
104 
106 
105 

108 
108 
108 
100 
86 

52 
49 

73 
73 

62 
73 

92 
98 

52 
51 

80 
71 

73 
66 
63 

64 
64 

65 
70 
66 
63 
65 

61 

67 
73 

67 

78 

82 

102 
92 
95 
97 
94 

94 
99 
101 
100 
94 

96 
89 

18th 

19th 

20th 

21st 

22d 

23d 

24th 

25th 

26th 

27th 

28th 

29th 

30th 

39 
40 
41 

41 
41 
40 
46 
49 

43 
48 
62 
46 

54 
52 

52 
48 
49 
50 
49 

53 

47 
53 
53 
52 

60 

76 
79 

74 
71 
76 
78 
76 

78 
67 
72 
76 

78 

77 

100 
104 

31st 

1 

1 

Average  1 
difference, 

21  daj 

s,  30.5°,  20  days,  25.0° 

20  days,  24.1° 

21  days,  29.4° 

23  days,  26.1° 

17daj 

's,  27.3° 

have  been  used  in  the  construction  of  the  graph  in  figure  16,  which 
shows  the  gradient  for  the  Santa  Catalinas,  the  average  gradient  for 
17  mountain  ranges  in  different  portions  of  the  world  (according  to 
Hann),  the  gradient  for  Pike's  Peak,  the  Sierra  Nevada,  and  also  the 
gradient  in  the  free  air,  as  determined  at  the  Blue  Hill  Observatory. 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


77 


Some  estimate  of  the  error  involved  in  basing  the  gradient  only  on 
minimum  readings  may  be  made  by  the  figures  presented  in  tables 
14  and  15.  These  tables  exhibit  the  only  daily  records  of  maximum  and 
minimum  temperatures  for  the  Catalinas  for  any  period  longer  than 
a  few  days.  The  average  maxima  and  minima  for  Marshall  Gulch  for 
the  months  of  June,  July,  and  August  have  been  contrasted  in  table  14, 
with  the  average  maxima  and  minima  for  the  Laboratory.  During 
June  the  apartness  of  the  minima  was  30.5°,  of  the  maxima  25.0°.  In 
July  this  relation  was  reversed,  the  apartness  of  the  minima  being 
24.1°,  that  of  the  maxima  29.4°,  while  for  August  the  two  were  more 
nearly  the  same,  the  apartness  of  maxima  being  26.1°,  of  maxima  27.3°. 
The  facts  that  the  minimum  temperatures  of  valley  and  mountain 
were  further  apart  than  the  maxima  were  during  June,  and  not  so 
far  apart  in  July  and  August,  may  be  connected  in  some  manner  with 
the  clear  dry  weather  of  June  and  the  rainy,  cloudy  character  of  July 
and  August.    However,  the  data  in  table  15,  showing  the  maximum 

Table  15. — Daily  record  of  maximum  and  minimum  temperatures  for  a  portion  of  June 
1912,  at  summit  of  Mount  Lemmon,  with  corresponding  data  for  the  Desert  Laboratory. 


Day  of  month. 

Minimum . 

Maximum. 

Day  of  month. 

Minimum. 

Maximum. 

M.  L. 

D.  L. 

M.L. 

D.L. 

M.L. 

D.L. 

M.L. 

D.L. 

June    7 

8 

9 

10 

11 

12 

"F. 

46 

46 

45 

40.5 

42 

41.5 

°F. 

"76" 
71 

°F. 
73 

77 
72 
67 
69 
69.6 

°F. 

"162' 
101 
100 

June  13 

14 

15 

16 

17 

18 

°F. 
43 
43.5 
45 

44 
41 
41 

op 
74 
71 
76 

78 
77 
72 

°F. 
68 
66 
69 
71 
70 
69 

°F. 
101 
104 
105 
102 
102 
106 

Average  difference  of  maxima,  33.9;   of  minima,  31.1. 

and  minimum  temperatures  on  Mount  Lemmon  in  June  1912,  indi- 
cate that  there  was  a  greater  apartness  of  maxima  than  of  minima 
when  these  data  are  averaged  and  contrasted  with  those  for  the  Desert 
Laboratory. 

It  would  require  a  much  fuller  mass  of  figures  than  are  in  hand  to 
make  any  attempt  at  an  explanation  of  the  differences  that  exist  in 
the  apartness  of  desert  and  mountain  maxima  and  minima  in  different 
localities  and  different  months.  For  the  present  purpose  it  is  instruc- 
tive to  average  the  entire  set  of  apartnesses  for  Marshall  Gulch  for 
June,  July,  and  August  1911,  which  gives  an  average  difference  of 
minima  of  80.7°,  and  of  maxima  of  81.7°.  In  other  words,  throughout 
a  series  of  several  months  there  is  doubtless  a  swamping  of  the  irregu- 
larity of  the  apartnesses  for  individual  months.  If,  then,  there  is  an 
average  equahty  between  the  apartness  of  maxima  and  minima — which 
is  to  say  that  there  is  an  equahty  of  daily  mean  range  of  temperature 
between  desert  and  mountain — it  would  indicate  that  the  minima  are 


78 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


just  as  good  data  on  which  to  base  an  estimation  of  the  gradient  as 
mean  temperatures  would  be.  Since  the  figures  used  in  constructing 
the  gradient  were  secured  in  all  months  from  April  to  October,  in 
several  years,  and  in  a  wide  diversity  of  locaUties — all  outside  the 
influence  of  cold-air  drainage — it  is  probable  that  the  gradient  here 
presented,  figure  16,  is  within  one  or  two  degrees  of  the  same  measure 
of  accuracy  that  could  be  secured  by  a  long  series  of  consecutive  read- 
ings of  maximum  and  minimum. 

Using  the  elevation  of  the  Desert  Laboratory  (2,663  feet)  as  a  base, 
the  actual  fall  of  temperature  between  that  locality  and  the  9,000-foot 
station  on  the  Santa  CataUnas  is  at  the  rate  of  4.11°  per  1,000  feet. 
The  gradients  between  the  several  mountain  stations  are  indicated  in 


FiQ.  17. — Graphs  showing  march  of  weekly  minimum  temperature  at 
Desert  Laboratory  (upper)  and  weekly  or  other  minimum  tempera- 
ture at  Marshall  Gulch  for  1911  (lower). 

figure  16,  on  which  it  will  be  seen  that  there  is  a  negative  gradient 
between  the  Desert  Laboratory  and  the  4,000-foot  station,  and  that 
the  gradient  between  4,000  and  5,000  feet  is  at  the  rate  of  10°  per  1,000 
feet.  From  5,000  feet  upward  the  gradients  are  more  uniform  and 
more  nearly  equal  between  the  successive  stations. 

Figures  are  given  by  Hann  *  for  the  gradients  of  temperature  on  a 
number  of  mountains  in  different  parts  of  the  world.  The  average 
values  for  17  extra-tropical  mountains  is  3.13°  F.  per  1,000  feet  (0.57°  C. 
per  100  m.).  The  only  mountains  in  the  western  United  States  for 
which  Hann  gives  figures  are  Pike's  Peak  and  Sierra  Nevada  (Colfax, 
Placer  County).    The  gradient  of  the  former  is  3.46°  per  1,000  feet 


*  Hann,  J. 
York,  1903. 


Handbook  of  Climatology.    Translation  by  R.  DeC.  Ward,  pp.,  243-246.     New 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  79 

(0.63°  C.  per  100  m.),  and  for  the  latter  4.12°  per  1,000  feet  (0.75°  C. 
per  100  m.).  The  gradient  for  Colfax  is  seen  to  be  almost  exactly- 
coincident  with  the  entire  gradient  for  the  Santa  Catalinas.  It  is 
interesting  to  note,  in  this  connection,  that  the  fall  of  temperature  in 
the  free  air  has  been  determined  at  the  Blue  Hill  Observatory  to  be 
2.5°  F.  per  1,000  feet,  which  is  far  more  gradual  than  any  of  the  moun- 
tain gradients  that  have  been  cited.  The  Blue  Hill  data  apply  only 
to  low  elevations,  but  are  in  substantial  agreement  with  figures  more 
recently  secured  in  the  free  air  at  Avalon,  Cahfornia.*  Seven  balloon 
ascensions  from  Avalon  to  elevations  of  18  km.  and  higher  showed  a 
mean  gradient  of  fall  in  the  first  3  km.  (9,842  ft.)  of  2.2°  per  1,000  feet. 
These  two  determinations  of  the  free-air  gradient  indicate  a  conserva- 
tism of  temperature  change  in  the  lower  atmosphere  as  compared 
with  the  changes  on  the  slopes  of  mountains. 

While  the  normal  temperature  gradient  is  of  profound  interest  from 
the  standpoint  of  pure  climatology,  it  is  nevertheless  of  subsidiary 
value  in  the  study  of  climate  in  relation  to  vegetation.  Its  chief  value 
is  as  a  basis  with  which  to  compare  the  differentiation  of  temperature 
conditions  originating  in  the  irregularities  of  topography  and  other 
causes.  In  later  pages  the  subsidiary  influences  upon  the  temperature 
gradient  will  be  discussed. 

THE  ABSOLUTE  MINIMUM  OF  WINTER. 

The  absolute  minimum  temperature  of  the  winter  was  secured  at 
5,300  feet  and  at  7,600  feet  for  four  winters,  and  during  the  winter  of 
1912  and  1913  was  secured  at  four  stations,  and  during  the  succeeding 
winter  at  10  stations,  differing  both  in  altitude  and  in  topographic 
location. 

The  winter  of  1912  and  1913  was  one  of  exceptional  severity  at 
Tucson — in  fact  throughout  the  extreme  southwestern  United  States — 
while  the  winter  of  1913  and  1914  was  one  of  the  customary  modera- 
tion. The  data  for  these  two  winters  are  calculated,  therefore,  to 
exhibit  the  extreme  and  the  average  conditions  of  winter  temperature 
for  stations  in  Arizona. 

The  minimum  temperature  readings  at  the  mountain  stations  are 
given  in  table  16;  and  in  table  17  are  given  the  minima  for  December, 
January,  and  February  of  the  same  years  for  a  selected  series  of  stations 
in  Arizona.  The  lowest  temperature  recorded  on  the  Santa  Catalinas 
in  1912-13  was  —6°  at  6,000  feet,  while  the  lowest  temperature  at 
the  highest  station,  at  7,600  feet,  was  —2°.  This  figure  should  be  con- 
trasted both  with  the  absolute  minimum  at  the  Arizona  Experiment 
Station,  6°,  and  with  that  at  the  office  of  the  Desert  Laboratorj^,  1°, 
as  well  as  with  that  for  Flagstaff,— 23°,  situated  in  northern  central 

*  Blair,  William  R.  Free-Air  Data  in  Southern  California,  July  and  August,  1913.  Mo. 
Weather  Rev.,  42:  410-426,  1914. 


80 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


Arizona  at  an  elevation  700  feet  lower  than  that  of  the  7,600-foot 
Station  in  Marshall  Gulch.  The  extremely  low  temperatures  at  the 
Arizona  Experiment  Station  and  the  office  of  the  Desert  Laboratory, 
as  contrasted  with  the  minimum  temperature  of  17°  at  the  Desert 


Table  16. 


-Absolute  minimum  temperatures  of  two  winters  at  stations  in 
Mountains  and  at  Tucson. 


Santa  Catalina 


Station. 

Location  of  station. 

1912-1913. 

1913-1914 

Arizona  Experiment  Station 
Office  of  Desert  Laboratory 

Near  bottom  of  Santa  Cruz  Valley . 

Edge  of  flood-plain  of  Santa  Cruz 

Valley                               

6 

1 

17 
13 

-6 

.5 
-2 

•• 

26 

22 

29 
27 

18 

18 
18 
6 

12 

15.5 

5 

5 

3 

Slope  of  hill,  335  feet  above  Santa 

4  000  feet 

Ridge,  in  Lower  Desert  Region 

Slope,  125  feet  above  floor  of  Soldier 
Canon           

5  000  R                

5  000  V 

Floor  of  Soldier  Canon,  in  Lower 
Encinal             .        

6  000  V 

Floor  of  Bear  Canon 

7  000                  

Ridge,  north  rim  of  Bear  Cafion,  in 

7  600                        

Bottom  of  Marshall  Gulch,  in  Fir 
Forest           

8  000 

Ridge,  north  rim  of  Marshall  Gulch, 
Pine  Forest                       

9  000 

North  slope,  below  summit  of  Mount 

Lemmon  in  heavy  Fir  Forest .... 

Top   of   fire-tower  on   summit   of 

9  225 

Laboratory,  are  to  be  accounted  for  through  the  operation  of  cold-air 
drainage.  On  the  coldest  night  in  January  1913  there  was  a  difference 
of  only  3°  between  the  temperatures  of  the  Santa  Cruz  Valley  and 
Marshall  Gulch,  5,400  vertical  feet  apart.    It  is  desired  here  not  so 

Table  17. — Absolute  minimum  temperatures  of  winter  months  for  two  winters  at  selected 
stations  in  Arizona,  together  with  the  absolute  winter  minima  at  7,600  feet  in  the  Santa 
Catalina  Mountains. 


station. 


1912-1913. 


Dec.      Jan.      Feb 


1913-1914. 


Dec.      Jan.      Feb, 


Santa  Catalinas,  7,600  feet 

Tucson,  2,390  feet 

Chlarson's  Mill,  7,200  feet 

Flagstaff,  6,907  feet 

Fort  Valley  Experiment  Station,  7,300  feet . 

Fort  Apache,  5,200  feet 

Fort  Huachuca,  5,100  feet 

Snowflake,  5,644  feet 


28 


28 


15.5 
26 


26 


—  1 
-13 

-  1 
23 

7 


2 

-  4 

14 

29 

9 


much  to  lay  emphasis  on  the  exceptional  coldness  of  the  Santa  Cruz 
valley,  but  on  the  exceptional  warmness  of  the  summit  of  the  Santa 
Catalinas,  as  contrasted  with  similar  elevations  in  Arizona  which  are 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  81 

very  dissimilar  in  their  topographic  location.  In  table  17  it  will  be 
seen  that  the  winter  minimum  at  Flagstaff,  in  northern  central  Arizona, 
was  21°  below  the  minimum  for  Marshall  Gulch,  although  Flagstaff  is 
located  700  feet  lower.  This  relation  is  similar  to  that  which  exists 
between  the  length  of  frostless  season  at  Marshall  Gulch  and  at  Flag- 
staff, and  is  due  to  the  facts  mentioned  in  that  connection  on  page  74. 
At  the  Fort  Valley  Experiment  Station,  located  in  the  vicinity  of 
Flagstaff  and  nearer  to  the  San  Francisco  Peaks,  the  winter  minimum 
was  4°  lower  than  at  Flagstaff.  At  Snowflake,  on  the  extensive  Mogol- 
lon  Plateau,  and  at  Fort  Apache,  in  the  cold-air  drainage  of  one  of  the 
main  forks  of  the  Salt  River,  there  were  also  minima  which  were 
respectively  9°  and  3°  lower  than  at  Marshall  Gulch,  although  these 
stations  are  respectively  2,000  and  2,400  feet  lower  than  Marshall 
Gulch.  Fort  Huachuca  is  located  at  the  base  of  the  Huachuca  Moun- 
tains in  such  a  manner  as  to  escape  cold-air  drainage  from  any  of  the 
large  cafions  of  that  range  of  mountains,  and  its  absolute  minimum  was 
6°  higher  than  that  of  Fort  Apache,  which  is  of  approximately  the  same 
elevation.  Chlarson's  Mill  is  situated  in  Frye  Caiion  in  the  Pinaleno 
(Graham)  Mountains,  surrounded  by  heavily  forested  slopes.  Its  loca- 
tion is  analogous  to  that  of  Marshall  Gulch,  being  similarly  situated 
in  an  isolated  desert  mountain  and  surrounded  by  heavily  forested 
slopes.  The  single  monthly  minimum  available  for  Chlarson's  Mill 
is  15°,  for  a  month  in  which  the  minimum  at  Tucson  was  18°,  while 
it  was  7°  for  the  Fort  Valley  Experiment  Station,  at  almost  the  same 
elevation  as  Chlarson's  Mill. 

An  inspection  of  the  absolute  minima  for  1913-14,  in  table  17,  will 
show  that  the  same  relations  hold  true  between  the  several  stations 
that  have  just  been  discussed.  The  winter  minimum  for  Tucson,  26°, 
was  much  higher  than  in  the  preceding  winter,  and  so  was  that  for 
Marshall  Gulch,  15.5°,  although  the  absolute  minimum  in  the  new 
station  on  the  fire  tower  at  Mount  Lemmon  was  3°,  and  in  the  heavy 
timber  on  the  north  face  of  Mount  Lemmon  was  5°. 

The  data  just  discussed  amply  bear  out  the  statement  that  the  lowest 
temperatures  of  winter  are  less  severe  on  the  Santa  Catalinas  than 
they  are  at  the  same  elevation  on  the  plateau  of  north-central  Arizona, 
and  even  less  severe  than  they  are  in  many  situations  of  lower  altitude. 
The  fragmentary  records  of  earher  years  at  Chlarson's  Mill,  which 
are  not  given  here  but  are  available  in  publications  of  the  Weather 
Bureau,  show  that  it  is  likewise  favored  by  lower  winter  temperatures 
than  are  the  plateau  stations  of  the  same  elevation  in  Arizona.  The 
length  of  the  frostless  season  has  just  been  shown  to  be  less  in  the  Santa 
Catalinas  and  in  the  Pinaleno  Mountains  than  at  Flagstaff.  In  short, 
the  indications  are  very  strong  that  all  phases  of  winter  temperature 
conditions  are  less  severe  on  the  small  and  isolated  desert  mountains 
than  on  the  plateaus  and  highlands  of  the  same  elevations  and  of  nearly 


82  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

the  same  latitude.  The  radiation  of  the  desert  valleys  and  the  diurnal 
convection  currents  of  warm  air  are  not  without  a  strong  ameliorating 
influence  on  the  climate  of  elevated  but  small  masses  of  land. 

DEPARTURES  FROM  THE  NORMAL  TEMPERATURE  GRADIENT  DUE  TO 
COLD-AIR  DRAINAGE. 

The  ideal  conditions  of  uniform  decrease  of  temperature  with  increase 
of  altitude  are  seldom  actually  encountered  in  nature,  at  least  not  in 
mountains  of  small  size  and  rugged  topography.  The  principal  factor 
which  brings  about  departures  from  the  normal  or  ideal  gradient  of 
fall  is  the  operation  of  cold-air  drainage,  or  inversion  of  temperatures. 
This  is  a  phenomenon  which  has  long  been  known  and  has  frequently 
been  discussed  with  respect  to  its  influence  on  vegetation.  In  an 
earlier  paper  *  the  writer  has  described  some  observations  of  tempera- 
ture inversions  at  the  Desert  Laboratory  and  in  the  Santa  Catalinas, 
and  has  pointed  out  the  causes  involved  in  making  cold-air  drainage 
much  more  pronounced  in  deserts  than  it  is  in  humid  and  forested  regions. 

The  scanty  vegetation  of  the  desert  subjects  its  soil,  rocks,  and  sands 
to  full  insolation  and  to  a  pronounced  heating  throughout  the  day. 
The  dark  rocks  of  Tumamoc  and  other  volcanic  hills  in  its  vicinity 
become  so  hot  during  the  long  clear  days  of  May  and  June  that  it  is 
impossible  to  hold  one's  hand  on  them  without  pain.  During  the  day 
there  is  a  constant  and  active  radiation  of  heat  from  the  rocks  and  soil, 
which  warms  the  lowest  layers  of  air  and  causes  a  convectional  heating 
of  the  lowest  portion  of  the  atmosphere.  Immediately  after  sunset 
the  warmed  surfaces  become  rapidly  cooler  and  the  rate  of  radiation 
is  quickly  reduced.  The  air  nearest  the  cooling  rocks  and  soil  becomes 
cooler  than  the  air  above  it,  and  consequently  begins  to  fall  by  gravity 
before  there  is  opportunity  for  it  to  mix  with  the  warmer  air  above. 
This  cooled  air  descends  from  hillsides  and  even  from  gentle  slopes 
and  soon  collects  in  valleys  and  depressions,  where  it  results  in  a  slowly 
or  rapidly  moving  mass  of  air  which  is  appreciably  cooler  to  the  senses 
than  is  the  air  of  the  slopes  or  hillsides.  The  inversion  of  temperature 
thus  caused  usually  reaches  its  maximum  during  the  first  half  of  the 
night,  although  this  is  determined  in  great  measure  by  the  size  of  the 
drainage  area. 

It  is  only  on  clear  and  still  nights  that  cold-air  drainage  operates 
in  the  most  pronounced  manner.  A  high  wind  will  disturb  the  flow 
or  completely  eliminate  it.  Heaiy  cloudiness  will  cause  the  rate  of 
radiation  to  lag  so  that  there  is  time  for  an  admixture  of  cool  and  warm 
air,  thereby  preventing  the  flow  of  cold  air  or  greatly  reducing  it. 

On  clear  nights  which  f  oUow  heavy  rains  the  inversion  of  temperature 
will  be  reduced  to  a  negligible  amount,  because  of  the  increase  of  the 
specific  heat  of  the  soil  brought  about  by  its  becoming  wet. 

*  Shreve,  Forrest.    Cold  Air  Drainage.    The  Plant  World,  15:  11(>-115,  1912. 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS. 


83 


It  has  been  shown  in  the  paper  to  which  reference  has  been  made 
that  the  Desert  Laboratory  is  situated  well  above  the  level  of  the 
cold-air  flow  of  the  Santa  Cruz  Valley,  335  feet  below  it.  The  greatest 
observed  difference  of  minimum  temperature  in  a  single  night  between 
the  Laboratory  and  the  Valley  was  24°,  and  the  greatest  difference 
between  the  mean  monthly  minima  of  the  two  localities  was  17.8°  for 
May.  During  the  humid  mid-summer  the  mean  monthly  difference 
falls  to  8°  and  9°  for  these  stations. 

Table  18. — Minimum  temperature  records  to  show  the  operation  of  cold-air  drainage  in  the 
open  vegetation  of  Soldier  Canon  and  Bear  Cafion  and  its  abeyance  in  the  heavy  forest  of 
Marshall  Gulch. 

In  each  case  one  record  is  from  the  floor  of  the  canon  and  the  other  from  its  slopes  or  rim.  The 
minus  differences  indicate  a  higher  temperature  on  the  floor  and  the  absence  of  cold-air 
drainage. 


Dates. 


Slope  or 
rim. 

Floor. 

Difference 

18 

18 

0 

49 

42.5 

6.5 

52 

44 

8 

45 

39 

6 

60 

60 

0 

63 

62 

1 

40 

40 

0 

38 

33 

5 

12 

6 

6 

47 

38 

9 

38 

34 

4 

62 

54 

-   2 

34.5 

33.5 

1 

30.6 

31.6 

-    1 

6 

15.6 

-10.5 

33.5 

61.6 

-18 

50.5 

49.5 

1 

48.5 

50.5 

-   2 

51.5 

51.6 

0 

29.5 

32.5 

-  3 

Soldier  Canon,  floor  at  4,900  feet,  elope  at  5,026  feet: 

Sept.  27,  1913,  to  May  16,  1914 

May  17  to  19.  1914 

May  20,  1914 

May  21  to  July  22,  1914 

July   23  to  28,  1914 

July   29  to  Aug.  8,  1914 

Aug.     9  to  Oct.  10,  1914 

Bear  Canon,  floor  at  6,000  feet,  rim  at  7,000  feet: 

Sept.  24  to  Sept.  26,  1913 

Sept.  27,  1913,  to  May  17,  1914 

May  18  to  19,  1914 

May  20  to  July  23,  1914 

July  24  to  27,  1914 

Marshall  Gulch,  bottom  at  7,600  feet,  rim  at  8,000  feet 

Sept.  25,  1913 

Sept.  26,  1913 

Sept.  27,  1913,  to  May  17,  1914 

May  18  to  July  24,  1914 

July   25,  1914 

July  26,  1914 

July  27,  1914 

July  28  to  Oct.  11,  1914 


The  vigor  of  cold-air  drainage  is  determined  not  only  by  the  condi- 
tions of  cloudiness  and  wind  but  also  by  the  size  and  nature  of  the  area 
from  which  the  cold  air  is  derived  and  by  the  character  of  the  valley 
bottom  through  which  it  moves.  In  the  Santa  Cruz  Valley  cold  air 
is  derived  from  an  area  of  more  than  1,000  square  miles,  resulting  in 
the  pronounced  low  temperatures  shown  in  tables  16  and  18.  The 
broad  level  trough  of  the  valley  is  conducive  to  a  slow  movement  of 
the  air,  and  the  nocturnal  minimum  is  usually  reached  during  the  last 
hours  of  darkness.  The  valleys  of  the  Salt  and  Gila  Rivers  are  larger 
than  the  valley  of  the  Santa  Cruz,  and  they  have  their  sources  in  still 
higher  mountains,  but  they  do  not  seem  to  possess  a  well-marked 


84  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

cold-air  drainage,  to  judge  by  the  minimum  temperature  records  for 
the  towns  on  the  lower  courses  of  these  rivers,  as  Florence  and  Phoenix. 
It  is  possible  that  in  traveling  long  distances  at  nearly  the  same  altitude 
the  cold  air  is  gradually  warmed  by  mixture  with  the  warm  air  above  it. 

During  the  winter  of  1913-14  and  the  summer  of  1914  thermometers 
were  exposed  in  the  Santa  Catalinas  so  as  to  give  a  basis  for  comparing 
the  cold-air  drainages  of  the  mountain  with  the  drainage  of  the  Santa 
Cruz  Valley  as  investigated  at  the  Desert  Laboratory.  Table  18  shows 
the  readings  of  instruments  placed  so  as  to  reveal  the  differences  of 
temperature  due  to  cold-air  drainage,  at  three  localities  of  different 
elevation.  The  first  locality  is  in  Soldier  Canon,  in  the  open  Encinal, 
where  readings  were  taken  on  the  floor  of  the  caiion,  at  4,900  feet  and 
on  its  slope  at  5,025  feet.  The  lowest  minima  of  the  winter  were 
identical  at  the  two  stations,  which  can  be  accounted  for  only  on  the 
possibility  of  the  stream  of  cold  air  having  become  so  deep  as  to  reach 
the  upper  thermometer,  or  else  on  the  possibility  that  the  lowest  mini- 
mum of  the  winter  occurred  on  a  cloudy  or  very  windy  night.  During 
three  intervals  in  the  arid  fore-summer  the  depression  of  the  temperature 
in  the  floor  of  the  caiion  was  6.5°,  8°,  and  6°  respectively,  whereas  through- 
out the  humid  mid-summer  the  depression  was  absent  or  negligible. 

The  regular  7,000-foot  station  is  located  on  the  rim  of  Bear  Caiion, 
and  the  data  from  it  may  be  compared  with  those  from  an  instrument 
placed  in  the  floor  of  the  caiion  1,000  feet  below.  This  station  may 
further  be  compared  with  the  6,000-foot  station  located  on  the  summit 
of  Manzanita  Ridge.  In  spite  of  the  difference  of  1,000  feet  in  the 
elevation  of  the  two  thermometers  in  Bear  Cafion,  the  lowest  tempera- 
ture of  the  winter  was  6°  in  the  Canon  and  12°  on  the  rim.  A  two-night 
interval  in  the  autumn  of  1913  gave  a  difference  of  5°  between  these 
stations,  due  to  air  drainage,  and  during  the  arid  fore-summer  of  1914 
differences  of  9°  and  4°  were  obtained.  During  the  three  particularly 
cloudy  and  rainy  nights  in  July  there  was  an  actual  reversal  of  the 
conditions  of  cold-air  drainage  and  a  manifestation  of  the  true  tempera- 
ture conditions  to  be  expected  from  altitude  alone,  the  rim  having  a 
minimum  2°  lower  than  the  floor.  Although  the  winter  minimum  of 
1913-14  was  6°  in  the  floor  of  Bear  Caiion  at  6,000  feet,  it  was  18°  on 
Manzanita  Ridge  at  6,000  feet.  This  difference  of  12°  between  two 
stations  at  the  same  altitude  is  as  great  as  should  be  expected,  under 
the  operation  of  the  normal  gradient  of  temperature,  between  localities 
3,468  vertical  feet  apart  (12°-^3.46°,  the  rate  of  fall  per  1,000  feet). 

Although  there  are  some  small  bodies  of  forest  on  the  walls  of  Bear 
Canon  and  many  scattered  trees,  nevertheless  the  surface  of  its  sides 
is  largely  occupied  by  cliffs  and  boulders  (see  plate  21),  and  these  are 
responsible  for  the  acute  operation  of  the  drainage  phenomenon. 

Several  preliminary  tests  had  shown  an  extremely  weak  manifesta- 
tion of  cold-air  drainage  at  the  heavily  forested  elevations  of  the  Santa 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  85 

Catalinas.  The  data  presented  in  table  18  for  the  rim  of  Marshall 
Gulch  at  8,000  feet,  and  for  the  Montane  Garden  at  7,600  feet,  bear 
out  the  results  of  the  preliminary  tests.  The  rim  was  10°  colder  than 
the  bottom  of  the  gulch  in  the  over- winter  period  and  18°  colder  in 
the  period  from  May  18  to  July  24.  The  5  one-night  readings  and  the 
readings  for  the  period  from  July  28  to  October  11  all  show  an  equality 
or  a  slight  difference,  more  often  a  difference,  with  the  temperature 
more  commonly  higher  in  the  bottom  of  the  Gulch  than  on  the  rim. 
In  other  words,  cold-air  drainage  is  in  abeyance  at  this  locality. 
Whether  this  is  invariably  the  case  can  only  be  stated  after  further 
instrumentation  and  after  complete  assurance  that  the  readings  have 
not  been  influenced  by  the  character  of  the  weather  during  the  nights 
of  lowest  temperature.  The  heavy  vegetation  of  the  Forest  region, 
together  with  the  high  humus  content  of  the  soil  and  the  litter  of  leaves 
by  which  it  is  covered,  all  miUtate  against  the  rapid  terrestrial  radia- 
tion in  which  cold-air  drainage  has  its  origin.  It  will  not  be  surprising, 
therefore,  to  find  that  the  phenomenon  is  either  very  weak  or  absent 
above  the  elevation  of  7,000  feet  in  such  portions  of  the  mountain  as 
are  forested.  The  elimination  of  cold-air  drainage  by  a  forest  cover 
can  take  place  only  in  small  mountains  which  are  forested  to  the  sum- 
mit. A  large  mountain  mass,  an  extremely  steep  mountain  side,  or 
an  extensive  area  lying  above  timber  line  will  cause  a  flow  of  cold  air 
down  through  forested  areas  below.  This  is  exemplified  at  the  San 
Francisco  Peaks,  Arizona. 

The  case  mentioned  in  which  cold-air  drainage  occasioned  a  differ- 
ence of  temperature  at  the  same  altitude,  which  was  the  equivalent  of 
nearly  3,500  feet,  probably  represents  its  maximum  effect.  The  differ- 
ence of  8°  in  Soldier  Canon  is  the  equivalent  of  an  altitudinal  difference 
of  about  2,200  feet. 

The  influence  which  cold-air  drainage  exerts  on  vegetation  is  regis- 
tered chiefly  in  the  shortening  of  the  season  of  vegetative  activity  on 
the  floor  of  a  canon  as  contrasted  with  its  sides.  This  effect  has  been 
repeatedly  observed  in  Bear  Canon,  where  the  oaks  on  the  floor  of 
the  canon  are  always  far  behind  the  individuals  on  the  canon  wall  in 
the  advancement  of  their  new  foHage  in  the  spring.  Likewise  in  the 
autumn  the  frost-kiUing  of  herbaceous  perennials  and  of  the  leaves  of 
Prunus,  Rhus,  and  Populusjamesii  takes  place  in  the  floor  of  the  caiion, 
while  the  herbaceous  plants  of  the  slopes  are  still  green  and  active. 
The  plants  on  the  caiion  floor  are,  in  other  words,  subjected  to  a  grow- 
ing season  similar  to  that  usually  found  at  a  much  higher  altitude. 

The  influence  of  cold-air  drainage  in  determining  the  distribution  of 
plants  is  likewise  marked.  It  is  not  wholly  responsible  for  the  fact 
that  mountain  species  extend  down  the  canons  to  lower  altitudes  than 
they  assume  on  slopes  or  ridges,  for  the  influence  of  ground  water  and 
soil  moisture  is  very  potent  in  this  connection.    The  occurrence  of  the 


86 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


highest  individuals  of  every  species — other  than  those  of  aquatic  or 
streamside  habitat' — on  or  near  the  summits  of  ridges,  and  their  invari- 
able absence  from  the  bottoms  of  canons  at  these  higher  elevations, 
are  to  be  attributed  to  the  absence  of  cold-air  drainage  from  the  ridges 
and  higher  slopes,  together  with  the  influences  grouped  in  the  ''factor" 
of  slope  exposure. 

SOIL  TEMPERATURE. 

In  the  autumn  of  1913  instruments  were  placed  at  the  6,000-  and  8,000- 
foot  stations  to  secure  the  absolute  winter  minimum  temperature  of 
the  soil  at  a  depth  of  3  cm.,  and  the  thermometers  were  maintained  in 


Table  19. — Minimum  temperatures  of  the  soil  and  of  the  air  at  3  elevations  in  the 
Santa  Catalina  Mountains  for  irregular  periods. 


Station. 


At  6,000  feet: 

Sept.  23  to  Sept.  27,  1913 

Sept.  28,  1913,  to  May  16,  1914 

May  17  to  19 

May  20  to  July  22 

July  23  to  27 

July  28  to  Oct.  10 

At  8,000  feet: 

Sept.  25,  1913 

Sept.  26,  1913 

Sept.  27,  1913,  to  May  17,  1914 

May  18 

May  19  to  July  24 

July  25 

July  26 

July  27 

July  28  to  Oct.  11 

At  7,600  feet: 

Sept.  25.  1913 


Air 

temperature. 


34.5 
30.5 

5 
39.6 
33.5 
60.5 
48.6 
51.5 
29.5 


Soil 
temperature. 


39 


+  5 
+10 
—   2 

+  1 
+  2 
+  2 


+  6.5 
+  6.6 
+25.0 
+  3.5 
+  7.6 
+18.5 
+  9.5 
+  5.5 
+  9.5 


place  and  read  at  irregular  intervals  during  the  summer  of  1914.  The 
object  in  placing  the  thermometers  at  so  slight  a  depth  was  to  obtain 
a  measure  of  the  activity  of  terrestrial  radiation  by  a  comparison  of  the 
superficial  minima  of  the  soil  and  the  atmospheric  minima.  The  ordi- 
nary type  of  Six's  thermometer  was  used,  buried  in  a  wooden  box  and 
covered  with  earth.  The  readings  secured  in  this  manner  and  the 
readings  of  atmospheric  minima  for  the  corresponding  periods  at  the 
same  stations  are  given  in  table  19. 

At  the  6,000-foot  station  the  soil  minima  are  higher  than  the  air 
minima  in  every  case  except  one,  the  over- winter  difference  being  10°. 
At  the  8,000-foot  station  all  of  the  9  readings  secured  show  a  higher 
minimum  for  the  soil.  The  over- winter  period  shows  a  difference  of 
25°,  and  the  night  of  July  25  shows  a  difference  of  18.5°.    The  readings 


CLIMATE  OF  THE  SANTA  CATALINA  MOUNTAINS.  87 

show  that  the  soil  temperatures  at  6,000  feet  were  cooler  in  general, 
in  terms  of  the  air  temperature,  than  were  those  of  the  8,000-foot 
station.  This  difference  is  not  to  be  attributed  to  the  difference  of 
elevation  so  much  as  to  the  naked  and  stony  character  of  the  soil  at 
the  6,000-foot  station  and  the  relatively  abundant  humus  and  litter  in 
the  surface  soil  at  8,000  feet.  In  short,  the  radiation  from  the  soil 
surfaces  in  the  Encinal  is  greater  than  it  is  in  the  Forest,  as  has  been 
already  discovered  from  the  difference  in  the  behavior  of  cold-air 
drainage  in  these  two  regions.  There  is  also  a  shght  indication  that 
the  differences  between  the  air  and  soil  minima  are  least  in  the  dry 
seasons  of  May  and  September,  which  is  again  in  keeping  with  the 
greater  radiation  exhibited  in  dry  soils  as  compared  with  wet  ones. 

On  the  night  of  September  25,  1913,  the  difference  between  the  air 
and  soil  temperatures  was  simultaneously  determined  on  the  rim  of 
Marshall  Gulch  at  the  8,000-foot  station  and  in  a  thicket  of  young  fir 
trees  in  the  bottom  of  the  gulch.  The  soil  remained  6.5°  warmer  than 
the  air  on  the  rim  of  the  gulch  and  9°  warmer  in  the  fir  thicket,  showing 
the  degree  to  which  a  heavy  cover  of  vegetation  retards  radiation  and 
conserves  the  warmth  of  the  soil.  On  this  night  the  air  temperature 
in  the  bottom  of  the  gulch  was  1°  lower  than  that  on  the  rim  (see 
table  18). 

One  of  the  most  striking  features  of  the  soil  minima  is  the  fact  that 
although  the  air  temperature  at  8,000  feet  fell  to  5°  in  the  winter  of 
1913-14,  the  soil  temperature  fell  only  to  30°.  This  means  that  in  the  open 
forest  on  the  rim  of  Marshall  Gulch  the  soil  must  have  been  very  shghtly 
if  at  all  frozen  in  the  winter  in  question,  which  was  apparently  a  winter 
of  about  average  severity.  During  the  same  winter  a  lower  absolute 
minimum  of  the  soil  was  recorded  at  6,000  feet  than  at  8,000  feet.  In 
shaded  situations  and  on  north  slopes  in  the  Fir  Forest  the  soil  undoubt- 
edly freezes  to  a  shght  depth.  Inasmuch  as  no  soil  temperatures  have 
been  secured  with  the  bulb  of  the  thermometer  in  contact  with  the 
soil,  and  no  readings  have  been  secured  at  a  greater  depth  than  3  cm., 
the  further  discussion  of  soil  temperature  conditions  in  these  moun- 
tains should  await  further  investigation. 


88  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


CORRELATION  OF  VEGETATION  AND  CLIMATE  IN  THE 
SANTA  CATALINA  MOUNTANS. 
The  earlier  chapters  of  the  present  paper  have  described  the  salient 
features  of  the  vertical  distribution  of  vegetation  in  the  Santa  Catalinas, 
and  also  some  of  the  principal  gradients  of  cUmatic  change.  Both  the 
vegetation  and  the  climate  have  been  shown  to  exhibit  progressive 
changes  with  increase  of  altitude,  and  these  changes  have  been  found 
to  undergo  hastening  or  retardation  under  the  influence  of  topographic 
irregularities.  It  will  be  the  object  of  the  following  pages  to  correlate, 
in  so  far  as  possible,  the  altitudinal  changes  of  vegetation  and  climate, 
in  an  effort  to  determine  roughly  some  of  the  physical  factors  which 
are  of  critical  importance  in  limiting  the  vertical  ranges  of  the  types 
of  vegetation  and  of  their  characteristic  species. 

THE  NORMAL  ALTITUDINAL  GRADIENT  OF  VEGETATION. 

Any  attempt  to  ascribe  vertical  limits  to  the  Desert,  Encinal,  and 
Forest,  or  to  state  the  vertical  limits  of  individual  species,  is  met  at 
once  by  the  omnipresent  importance  of  slope  exposure  in  determining 
these  limits.  The  altitudinal  range  of  vegetations  and  species  may  be 
determined  by  examining  only  slopes  of  south  exposure,  or  only  those 
of  north  exposure,  and  the  two  examinations  would  agree  closely  as 
respects  the  vertical  ranges,  but  would  disagree  by  approximately 
1,000  feet  with  respect  to  the  upper  and  lower  limits  of  the  vegetations 
or  species.  It  is  impossible  to  determine  the  normal  character  of  vege- 
tation at  a  given  altitude  by  seeking  level  ground,  for  it  will  be  found 
only  in  the  flood-plains,  subject  to  the  influence  of  a  high  soil  moisture, 
or  on  a  ridge,  subject  to  equally  special  conditions.  It  is  also  impos- 
sible to  visit  adjacent  valleys  or  plateaus  lying  at  the  same  elevation 
and  to  find  on  them  vegetation  which  is  subject  to  the  same  climatic 
and  soil  conditions.  For  some  purposes  it  is  desirable  to  consider  the 
vertical  stages  of  vegetation  under  ideal  conditions,  as  affected  by 
altitude  without  the  complications  due  to  topographic  features.  It 
is  then  possible  to  hypothecate  a  nonn  of  vertical  stages  of  vegetation 
by  averaging  the  altitude  of  any  given  limit  as  separately  determined  on 
north  and  south  slopes,  or  it  is  possible  to  take  into  consideration  only 
the  altitudinal  changes  of  south  slopes  or  of  north  slopes,  taken  alone. 

It  has  been  shown  that  the  influence  of  topography  on  the  vegetation 
is  chiefly  (sometimes  solely)  to  carry  the  common  types  of  vegetation 
above  or  below  the  elevations  at  which  they  are  universal.  The 
influence  of  topography  on  the  gradients  of  climate  is  of  the  same  char- 
acter; the  topographic  relief  causes  no  wholly  new  factors  to  come  into 
play,  but  serves  merely  to  carry  the  physical  conditions  of  the  Desert 
into  the  Encinal,  for  example,  or  to  bring  the  conditions  of  the  Forest 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  89 

down  into  the  Encinal.  The  physical  factors  which  underlie  the  effects 
of  topography  are,  then,  to  be  considered  simply  as  special  cases  of  the 
same  influences  that  are  grouped  in  the  effects  of  altitude  itself.  It 
is  desirable,  nevertheless,  in  studying  the  correlation  of  climate  and 
vegetation  to  consider  separately  the  normal  gradient  of  vegetation 
and  the  departures  from  the  normal  gradient. 

THE  VERTICAL  DISTRIBUTION  OF  INDIVIDUAL  SPECIES. 

The  student  of  vegetation  too  often  loses  sight  of  the  fact  that  vege- 
tation is  composed  of  individual  species  of  plants  and  that  the  behavior 
of  the  vegetation  is  a  function  of  the  behaviors  of  these  species.  After 
our  review  of  the  vegetation  of  the  Santa  Catalinas,  and  in  connection 
with  the  discussion  of  its  control  by  climatic  factors,  it  is  necessary 
to  consider  the  vertical  distribution  of  the  individual  species  in  relation 
to  the  physical  conditions  of  the  mountain. 

There  are  no  species  of  plants  which  grow  spontaneously  both  at 
the  base  and  the  summit  of  the  Santa  Catalina  Mountains,  except  a 
few  palustrine  forms  of  Carex  and  Juncus.  The  total  range  of  physical 
conditions  through  the  6,000  feet  of  elevation  here  involved  is  so  great 
that  no  native  plant  possesses  the  power  of  accommodation  to  the 
complete  gamut  of  Desert,  Encinal,  and  Forest.  Indeed,  very  few 
plants  range  through  half  of  the  entire  gradient  of  conditions,  in  any 
portion  of  it.  The  species  which  exhibit  the  widest  belts  of  vertical 
distribution  are  to  be  found  in  the  most  dissimilar  habitats  at  the  lower 
and  upper  edges  of  their  ranges,  which  indicates  that  these  species  are 
not  really  capable  of  existence  through  2,000  or  3,000  vertical  feet  of 
the  climatic  gradient  under  the  same  conditions  of  topographic  loca- 
tion, slope  exposure,  and  insolation.  In  fact,  a  close  analysis  of  the 
habitats  occupied  by  characteristic  plants,  in  connection  with  their 
vertical  ranges,  indicates  that,  below  6,000  or  7,000  feet,  no  plants 
outside  the  desert  succulents  and  semi-succulents  range  through  more 
than  1,000  to  1,500  feet  in  habitats  of  the  same  topographic  character. 
At  higher  elevations  a  number  of  common  plants  extend  more  than 
1,500  feet  in  situations  of  the  same  character,  as  for  example  Pinus 
arizonica,  which  ranges  through  nearly  twice  that  altitude  on  dry 
southern  slopes. 

A  vertical  range  of  4,700  feet  is  exhibited  by  Vitis  arizonica,  which 
occurs  in  several  arroyos  and  canons  at  3,000  feet  and  is  found  in  the 
same  habitat  throughout  the  Desert  and  Encinal  regions  of  the  moun- 
tain, reaching  its  highest  observed  station  at  7,700  feet  in  a  steep  dry 
arroyo  in  the  Pine  Forest.  Although  the  habitat  of  Vitis  is  superficially 
identical  throughout  its  range,  it  is  found  at  3,000  to  5,000  feet  only 
in  the  largest  arroyos,  in  which  it  is  able  to  draw  upon  much  greater 
and  more  constant  supplies  of  soil  moisture  than  are  available  in  the 
small  arroyos  to  which  it  is  confined  at  the  upper  edge  of  its  range. 


90  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

RoUnia  neomexicana  ranges  through  3,800  feet,  from  its  lowest  occur- 
rence on  flood-plains  near  constant  water  at  5,300  feet,  to  its  highest 
occurrence  on  dry  ridges  near  the  summit  of  Mount  Lenomon  at  9,100 
feet.  On  the  higher  mountains  of  southern  Arizona  this  species  ascends 
to  over  10,000  feet.  Amorpha  californica,  after  the  manner  of  Vitis, 
ranges  3,500  feet  from  moist  arroyos  at  4,200  feet  to  dry  ones  at  7,700 
feet.  Agave  palmeri  ranges  from  dry  slopes  of  east  or  west  exposure  at 
3,200  feet  to  open  ridges  and  crevices  of  rock  at  7,400  feet,  a  belt  of 
4,200  feet.  Among  the  species  which  reach  neither  the  Bajada  nor 
the  top  of  the  mountain  there  are  no  others  with  vertical  extensions 
of  more  than  4,000  feet.  An  extreme  range  of  3,700  feet  is  possessed 
by  Juniperus  pachyphloea,  from  northern  slopes  at  4,200  feet  to  ridges 
at  7,900  feet.  Nolina  microcarpa  extends  from  3,750  feet  on  north 
slopes  to  7,200  feet  in  open  pine  forest,  a  range  of  3,450  feet,  and  Dasy- 
lirion  wheeleri  from  3,600  feet  to  6,600  feet,  on  opposing  slopes,  a  range 
of  3,000  feet. 

Among  other  plants  which  occur  chiefly  in  the  Encinal  Region  there 
are  none  with  vertical  ranges  in  excess  of  3,000  feet,  few  in  fact  approach 
that  range.  Pinus  cembroides  extends  from  north  slopes  at  5,000  feet 
to  open  rocky  ridges  at  7,800  feet,  a  range  of  2,800  feet;  Agave  schottii 
ranges  from  3,700  to  6,000  feet,  a  belt  of  2,300  feet;  Garrya  wrightii 
ranges  from  4,300  to  6,500  feet,  an  extent  of  2,200  feet;  and  Quercus 
emoryi  extends  from  north  slopes  at  4,300  feet  to  south  slopes  at  6,200 
feet,  a  vertical  range  of  1,900  feet. 

Among  the  plants  which  have  their  lowest  occurrence  in  the  flood- 
plains  of  the  Encinal  and  their  principal  range  through  the  Forest 
Region,  a  number  have  vertical  belts  of  occurrence  of  more  than  3,000 
feet.  Pinus  arizonica  itself  is  found  through  3,300  feet  and  its  upper 
limit  is  determined  only  by  the  height  of  the  mountain.  Pseudotsuga 
mucronata  is  found  through  3,100  feet,  and  is  also  terminated  by  the 
summit  of  the  mountain.  Quercus  hypoleuca  and  Quercus  reticulata  are 
found  through  nearly  3,000  feet,  and  this  extent  of  vertical  range  is 
attained  by  a  large  number  of  herbaceous  perennials  of  the  Upper 
Encinal  and  Forest. 

The  Desert  species  which  are  encountered  at  the  foot  of  the  mountain 
do  not  begin  their  vertical  ranges  at  that  point,  and  statements  of  the 
elevations  which  they  reach  on  the  mountain  are  not  to  be  compared 
with  figures  for  the  ranges  of  Encinal  and  Forest  plants.  Mamillaria 
grahami  reaches  the  attenuated  limit  of  its  occurrence  at  7,000  feet, 
Echinocactus  wislizeni  at  5,600  feet,  Fouquieria  splendens  at  5,600  feet, 
Opuntia  versicolor  at  5,500  feet,  and  Carnegiea  gigantea  at  5,100  feet. 
Very  few  other  species  of  the  bajadas  and  desert  hills  are  found  above 
4,700  feet. 

Among  the  most  restricted  vertical  ranges  of  any  plants  which  reach 
neither  the  foot  nor  the  summit  of  the  mountain  are  those  of  Quercus 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  91 

oblongifolia  and  Vauquelinia  californica.  If  we  except  the  occurrence 
of  the  former  in  the  beds  of  Sabino  and  Ventana  canons  at  3,000  to 
3,200  feet,  its  lowest  occurrence  on  slopes  is  at  3,900  feet  and  its  highest 
at  5,600,  a  range  of  1,700  feet;  while  Vauquelinia  ranges  from  3,900  to 
5,500  feet,  a  vertical  range  of  only  1,600  feet.  These  limits  also  apply 
very  nearly  for  Erythrina  flabelliformis,  Ingenhousia  triloba,  and  several 
shrubs  and  shrublets,  and  are  only  slightly  exceeded  by  the  range  of 
Quercus  emoryi,  which  has  already  been  stated  to  be  1,900  feet. 

Certain  species  of  plants  are  confined  to  arroyos  throughout  their 
vertical  ranges,  as  are  Vitis  arizonica,  Amorpha  californica,  Platanus 
wrightii,  and  Juglans  rupestris;  or  are  found  chiefly  in  arroyos,  as 
Cupressus  arizonica  and  Acer  interior.  The  great  majority  of  trees, 
shrubs,  and  shrublets,  as  well  as  the  semi-succulents  (such  as  Agave, 
Yucca,  Nolina,  and  Dasylirion),  are  found  on  slopes  and  ridges  in  at 
least  some  portions  of  their  ranges,  or  are  chiefly  found  there.  The 
oaks,  the  deciduous  trees,  and  most  of  the  shrubs  may  be  found  along 
arroyos,  or  in  flood-plains  at  elevations  from  500  to  1,000  feet  below 
the  level  at  which  they  become  common  components  of  the  slope  vege- 
tation. The  semi-succulents,  like  the  succulents,  are  rarely  found  in 
arroyos,  although  they  may  grow  very  close  to  them  or  may  be  found 
in  dry  flood-plains.  Of  all  species  not  confined  to  arroyos,  their  lowest 
occurrences  are  generally  to  be  sought  on  north  slopes  or  in  arroyos 
at  even  lower  elevations,  and  their  highest  occurrences  are  to  be  sought 
on  south  or  southwestern  slopes  or  (particularly  in  the  case  of  cacti) 
on  rocky  ridges.  At  the  vertical  center  of  the  distributional  range  of 
these  species  they  may  be  found,  as  a  rule,  on  slopes  of  every  exposure, 
and  perhaps  in  flood-plains  as  well,  particularly  in  the  case  of  the  ever- 
green oaks.  The  exceptions  to  the  rule  are  Quercus  oblongifolia,  which 
is  commoner  on  south  slopes  than  on  north  ones  at  all  parts  of  its 
vertical  range  except  the  very  lowest,  and  Pinus  chihuahuana,  which 
is  rarely  found  on  north  slopes  at  any  part  of  its  range,  even  its  lowest 
occurrences  being  on  south  slopes  or  on  an  approximate  level. 

PHYSICAL  FACTORS  INVOLVED  IN  THE  DETERMINATION  OF  THE  NORMAL 
ALTITUDINAL  GRADIENT  OF  VEGETATION. 

In  order  to  overlook  for  the  moment  all  of  the  subsidiary  influences 
which  cause  local  disturbance  of  the  vegetistic  gradient  let  us  consider 
that  the  southern  slopes  at  all  elevations  are  representative  of  the 
normal  altitudinal  changes  of  vegetation,  and  let  us  then  consider 
some  of  the  differences  of  physical  conditions  that  accompany  the 
ascent  from  3,000  to  9,000  feet.  The  differentiations  of  vegetation 
which  we  are  accustomed  to  designate  as  ''due  to  altitude"  are  actually 
due  to  three  groups  of  physical  factors:  (a)  moisture  factors,  (b) 
temperature  factors,  (c)  light  factors.  It  has  been  customary  to  regard 
atmospheric  pressure  as  a  negligible  agency  in  relation  to  plants,  but 


92  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

there  is  no  work  known  to  the  writer  which  proves  or  disproves  this 
view.  The  Hght  factors  have  been  but  Httle  investigated  and  may 
prove  to  occupy  a  role  of  great  importance.  In  spite  of  the  funda- 
mental physiological  role  of  light,  it  is  more  than  probable,  however, 
that  this  factor  plays  a  less  important  part  in  influencing  the  distri- 
bution of  plants  than  do  moisture  and  temperature. 

MOISTURE  FACTORS. 

In  the  case  of  a  mountain  which  arises  from  an  arid  region  to  a 
considerable  height,  the  moisture  factors  are  of  critical  importance  in 
controUing  the  vertical  distribution  of  plants.  This  group  of  factors 
may  be  defined  as  those  which  have  to  do  with  the  maintenance  of  a 
close  degree  of  equaUty  between  the  daily  intake  and  outgo  of  water 
through  the  plant.  The  description  of  rainfall  and  soil  moisture  con- 
ditions in  the  Santa  Catalinas  has  indicated  the  nature  of  the  water 
supply  for  plants,  and  the  data  on  atmospheric  evaporation  have  shown 
the  collective  force  of  the  chief  of  those  ultimate  external  factors 
which  determine  the  water  loss  of  plants.  During  the  humid  seasons 
the  ratio  of  water  available  to  water  lost  is  such  as  to  make  conditions 
favorable  for  all  plants.  During  the  most  acute  periods  of  aridity  the 
value  of  this  ratio  becomes  an  item  of  the  first  moment. 

The  soil  moisture  data  given  in  an  earlier  section  are  from  too  slight 
a  depth  to  indicate  the  possible  supplies  for  trees  and  the  largest  shrubs. 
They  are  nevertheless  from  a  depth  which  is  freely  exploited  by  the 
roots  of  perennial  plants,  and  it  is  more  than  likely  that  they  bear  a 
rather  definite  ratio  to  the  moisture  at  greater  depths. 

Since  the  arid  fore-summer  is  the  portion  of  the  year  in  which  the 
maintenance  of  an  equilibrium  between  intake  and  outgo  of  water  is 
most  difficult,  it  is  instructive  to  determine  their  relation  for  this  season 
at  the  different  altitudes.  This  may  be  done  by  determining  the  ratio 
of  evaporation  (in  terms  of  cubic  centimeters  per  day)  to  soil  moisture 
(in  percentage  of  dry  weight),  using  the  average  daily  evaporation  of 
the  arid  fore-summer,  and  the  average  soil  moisture  of  the  arid  fore- 
summer  at  15  cm.  These  ratios  are  exhibited  in  table  20.  The  approx- 
imate ranges  of  the  ratios  are  1  to  25  for  the  Forest,  20  to  35  for  the 
Encinal,  and  35  to  50  for  the  Desert.  If  evaporation  data  had  been 
secured  at  9,000  feet  the  value  of  the  ratio  for  that  elevation  would 
have  been  less  than  unity. 

The  values  of  the  ratio  of  evaporation  to  soil  moisture  afford  a 
concise  expression  of  the  major  conditions  which  affect  the  water 
relations  of  plants,  and  they  demonstrate  the  wide  divergence  of  these 
conditions  in  the  desert  valleys  and  on  the  forested  mountain  summits 
during  the  arid  fore-summer.  The  average  daily  evaporation  rate 
has  been  shown  (fig.  12)  to  fall  during  the  humid  mid-summer  to  half 
the  amount  during  the  arid  fore-summer.    The  soil  moisture  is  like- 


CORRELATION  OF  VEGETATION  AND  CLIMATE,  93 

wise  increased  in  the  former  season  to  an  amount  that  would  greatly 
reduce  the  values  of  the  ratio  if  determined  for  the  humid  mid-summer. 
The  ratio  of  evaporation  to  soil  moisture  is  not  in  itself  a  full  index 
of  the  comparative  aridity  of  Desert,  Encinal,  and  Forest,  for  the  con- 
ditions expressed  by  the  ratio  are  of  much  longer  duration  at  3,000 
feet  than  at  8,000  feet.  The  shortening  of  the  arid  fore-summer  from 
16  weeks  at  3,000  feet  to  7  weeks  at  8,000  feet  (see  fig.  2)  signifies  that 
the  most  severe  drought  conditions  of  the  year  are  more  than  twice 
as  prolonged  at  the  lowest  elevation  as  compared  with  the  uppermost. 
It  is  necessary  here  to  bear  in  mind  that  the  effects  of  drought  on  plants 
are  cumulative,  and  that,  for  example,  a  period  with  a  given  set  of 
conditions  of  increasing  aridity  which  endures  for  16  weeks  may  be 
twice  as  fatal  or  deleterious  as  a  period  that  lasts  for  14  weeks.  For 
purposes  of  general  climatic  description,  however,  the  values  of  the 
ratio  of  evaporation  to  soil  moisture  multiplied  by  the  duration  of  the 
arid  fore-summer  may  be  taken  as  an  index  of  the  aridity  of  the  several 
elevations  (see  table  20) . 

Table  20. — Average  daily  evaporation  (E)  and  the  moisture  of  the  soil  (SM),  together  with 
the  ratio  of  evaporation  to  soil  moisture  [  -^-j-^  I  for  north  and  south  exposures  at  six  eleva- 
tions in  the  Santa  Catalina  Mountains  for  the  arid  fore-summer  of  1911. 


Elevation. 

Expo- 
sure. 

Vegetation. 

E 

SM 

E 
S  M 

Duration  of 

arid 
fore-summer. 

3,000  feet 

4,000  feet 

4,000  feet 

5,000  feet 

5,000  feet 

6,000  feet 

6,000  feet 

7,000  feet 

7,000  feet 

8,000  feet 

8,000  feet 

S 

S 
N 

s 

N 

s 

N 
S 

N 

s 

N 

Desert 

Desert 

Encinal 

Encinal 

Encinal 

Encinal 

Forest 

Forest 

Forest 

Forest 

Forest 

101.1 
80.4 
82.7 
61.7 
74.4 
59.4 
56.1 
62.8 
49.9 
29.3 
29.4 

2.0 
2.0 
2.5 
3.1 
3.5 
1.8 
3.5 
2.6 
5.5 
7.4 
11.3 

50.5 

40.2 

33.1 

19.9 

21.3 

33.0 

16.0 

24.1 

9.1 

3.9 

2.6 

16 
14 
13 

11 
9 

7 

The  ratio  of  evaporation  to  soil  moisture  comprises  a  measurement 
of  all  the  external  factors  which  affect  the  water  relations  of  plants, 
except  the  influence  of  radiant  energy  on  transpiration  and  the  possible 
effects  of  soil  temperature  on  this  function.  It  is  accordingly  unneces- 
sary to  give  further  consideration  to  rainfall,  which  is  not  in  itself  a 
factor  for  vegetation,  at  least  in  such  a  region  as  Arizona.  If  any 
differences  existed  between  the  seasonal  distribution  of  rainfall  at 
different  elevations  in  the  Santa  Catalinas  the  fact  would  be  of  great 
importance  to  the  vegetation,  but  only  in  the  effect  it  would  have  on 
the  annual  march  of  the  soil  moisture  conditions.  The  evidences  of 
observation  and  instrumentation  have  shown  that  the  major  drought 


94  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

periods  of  the  Desert  are  rarely  broken  on  the  mountain  by  rainfall 
of  significant  amount. 

The  records  of  rainfall  for  8  years  show  that  the  summer  rain  at 
8,000  feet  may  be  from  1.9  to  3.5  times  as  great  as  that  at  3,000  feet 
(see  table  4).  The  average  of  the  8  years  shows  the  summer  rain  at 
8,000  feet  to  be  about  2.4  times  that  on  the  Desert.  The  average  of 
a  longer  series  of  years  will  probably  approximate  this  amount  and 
the  securing  of  the  winter  precipitation  would  probably  make  little 
difference  in  the  proportion. 

The  very  conditions  of  low  evaporation  which  favor  the  water  rela- 
tions of  the  plants  in  the  Forest  region  are  also  favorable  to  the  preser- 
vation of  the  moisture  of  the  soil.  The  effect  of  the  winter  rains  upon 
the  soil  moisture  of  the  Forest  is  accordingly  carried  forward  many 
weeks  (see  table  7  for  soil  moisture  at  9,000  feet  after  6  weeks  without 
rain) .  The  slow  melting  of  snow  on  north  slopes  still  further  prolongs 
the  effect  of  winter  precipitation.  These  causes  underlie  the  vernal 
activity  of  herbaceous  plants  in  the  Forest  region  (see  p.  29)  and  the 
growth  by  trees  of  the  Forest  region  during  the  arid  fore-summer. 

TEMPERATURE  FACTORS. 

The  role  played  by  temperature  in  differentiating  the  vegetation  of 
the  various  altitudes  of  the  Santa  Catahnas  is  by  no  means  so  simple 
a  matter  as  that  played  by  moisture  conditions,  and  is  far  from  being 
capable  of  expression  in  a  concise  mathematical  form.  It  has  already 
been  shown  that  the  frostless  season  decreases  from  a  length  of  40 
weeks  on  the  Desert  to  a  length  of  19  weeks  in  the  Forest  at  7,600  feet 
(see  tables  10  and  11,  and  fig.  2),  and  that  the  temperature  has  an 
average  apartness  of  26°  between  Desert  and  Forest  (see  table  13). 
No  instrumentation  has  been  carried  on  which  would  establish  the 
quantitative  nature  of  the  difference  between  other  phases  of  the  tem- 
perature conditions.  The  shortening  of  the  growing  season  with  in- 
crease of  altitude,  and  the  concomitant  lowering  of  the  temperatures 
of  this  season,  are  factors  of  great  moment  to  the  vegetation,  but  no 
work  has  been  done  to  establish  the  precise  temperature  and  tempera- 
ture-duration requirements  of  any  species  of  plants.  The  shortness  of 
the  growing  season  in  the  Forest  and  the  coldness  of  the  nights  of 
mid-summer  (40°  to  50°,  see  table  14)  are  both  hostile  to  growth 
activity  and  may  well  be  limiting  factors  in  the  upward  distribution 
of  many  species  of  the  Encinal. 

The  temperature  conditions  of  winter  are  equally  important  with 
those  of  summer  in  underlying  the  limitation  of  species  and  vegetations, 
and  their  r61e  may  be  played  independently  from  that  of  the  summer 
temperature  conditions,  or  the  two  may  play  conjointly  upon  the 
same  species  at  the  same  altitude.  Among  the  various  phases  of  winter 
temperature  conditions  are  the  length  of  the  period  subject  to  frost, 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  95 

the  number  of  days  with  freezing  temperature,  the  number  of  consecu- 
tive days  or  hours  of  freezing,  and  the  absolute  minimum  reached. 
Any  two  or  more  of  these  phases  may  operate  conjointly  to  influence 
a  plant,  and  the  temperature  preceding  a  particular  constellation  of 
conditions  may  enhance  the  harmful  effects  of  those  conditions.  In 
fact  the  general  weather  conditions  accompanying  or  following  a  given 
phase  of  temperature  may  determine  the  full  effect  of  the  temperature. 

In  those  cases  in  which  plants  are  killed  by  the  action  of  low  tem- 
peratures the  most  important  factor  to  be  considered  is  the  actual 
duration  of  the  period  during  which  the  plant  is  subjected  to  tempera- 
tures below  32°.  Secondary  to  this  are  the  considerations  of  the  amount 
of  precooling  received  by  the  plant,  the  actual  minimum  temperature 
to  which  it  was  taken,  the  condition  of  the  soil  and  the  atmosphere 
during  the  freezing,  and  the  nature  of  the  weather  subsequent  to  it. 
In  a  previous  paper*  the  writer  has  called  attention  to  the  manner  in 
which  the  most  critical  phase  of  low  temperature  conditions  increases 
in  severity  with  increase  of  altitude.  Even  on  the  coldest  winter  days 
the  temperature  on  the  Desert  never  fails  to  rise  above  32°  during  the 
mid-day.  The  lowest  temperatures  of  winter  are  invariably  accom- 
panied by  a  clear  sky,  and  the  days  preceding  and  following  very  cold 
nights  are  clear.  A  cloudy  or  rainy  period  is  always  accompanied  by 
more  moderate  temperatures,  as  is  shown  by  the  rarity  of  snow  on  the 
Desert  itself.  The  longest  duration  of  a  shade  temperature  below 
freezing,  in  the  ten-year  records  of  the  Desert  Laboratory,  is  19  hours. 
It  frequently  happens  that  a  duration  of  6  hours  is  the  greatest  for 
an  entire  winter.  On  ascending  the  mountains  the  length  of  the  most 
prolonged  period  of  freezing  becomes  greater  until  an  altitude  is 
reached  at  which  there  are  occasional  winter  days  when  the  air 
temperature  does  not  rise  above  freezing.  At  this  altitude  there  is 
a  sudden  increase  in  the  maximum  number  of  hours  of  frost  from 
22  or  23  hours  to  a  length  of  40  to  45  hours.  Such  a  sudden  in- 
tensification in  the  duration  of  a  critical  climatic  condition  causes 
this  condition  to  operate  more  sharply  in  the  limitation  of  plant 
distribution  than  is  the  case  with  conditions  that  exhibit  the  usual 
form  of  slowly  graduated  change. 

No  winter  thermograph  records  have  been  secured  in  the  Santa 
Catalinas,  and  it  is  therefore  impossible  to  state  the  exact  altitude  at 
which  this  sudden  intensification  of  the  frost  factor  becomes  manifest. 
It  is  probable  that  it  lies  at  about  4,500  feet.  The  exposure  of  plants 
to  insolation  may  often  save  them  from  the  effects  of  an  air  tempera- 
ture of  less  than  32°,  and  in  the  case  of  succulents  the  temperature  to 
which  their  tissue  is  raised  during  the  insolation  of  the  preceding  day 
will  shorten  the  period  of  freezing  for  them.    These  subsidiary  matters 

*  Shreve,  Forrest.  The  Influence  of  Low  Temperatures  on  the  Distribution  of  the  Giant 
Cactus.    The  Plant  World,  14:  136-146,  1911. 


96  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

affect  the  exact  altitude  at  which  the  maximum  number  of  freezing 
hours  may  operate  in  the  hmitation  of  a  given  species;  and  the  exact 
topographic  character  of  the  location  of  an  individual  plant  may  also 
affect  the  operation  of  this  factor. 

The  experimental  work  of  the  writer  has  shown  that  a  duration  of 
more  than  18  hours  of  freezing  temperature  is  fatal  to  Carnegiea  and 
that  Opuntia  versicolor  and  Echinocereus  polyacanthos  are  capable  of 
withstanding  durations  of  66  hours.  The  limitation  of  Carnegiea  is 
apparently  due  to  the  operation  of  this  factor.  Its  occurrence  becomes 
confined  to  south  slopes  at  4,000  feet  and  it  becomes  less  and  less 
abundant  from  that  elevation  up  to  4,500  feet.  One  of  the  highest 
individuals  at  the  latter  elevation  is  protected  by  a  rock  on  its  north 
side,  above  the  summit  of  which  the  cactus  now  projects  for  8  inches. 
This  projecting  top  was  badly  frosted  on  its  north  side  in  the  severe 
winter  of  1912-13,  while  the  north  side  of  the  plant  below  the  summit 
of  the  rock  was  uninjured.  A  small  Carnegiea  (18  inches  high)  has 
been  discovered  in  Soldier  Caiion  at  5,100  feet.  It  grows  on  the  south 
side  of  a  low  rock,  and  its  location  is  on  the  steep  south  slope  which 
terminates  a  long  ridge  between  two  main  branches  of  the  canon.  The 
plant  is  here  well  protected  from  the  cold-air  flow  of  the  caiion  and  is 
subjected  to  the  full  insolation  of  the  short  winter  days.  It  showed 
some  slight  effects  from  the  exceptionally  cold  winter  just  referred  to, 
but  succeeded  in  recovering  from  them.  In  the  early  arid  fore-summer 
of  1911  the  writer  transplanted  a  young  Carnegiea  3  inches  high  from 
the  base  of  the  mountain  to  the  vicinity  of  the  6,000-foot  station  on 
Manzanita  Ridge.  The  cactus  was  placed  on  the  southwest  side  of  a 
rock,  with  a  large  plant  of  Arctostaphylos  northeast  of  it,  and  occupied 
a  location  near  the  summit  of  the  ridge.  The  plant  was  watered 
several  times  in  order  to  help  it  to  become  established,  but  was  not 
assisted  after  the  commencement  of  the  summer  rains.  It  success- 
fully passed  the  winter  of  1911-12;  it  made  gains  in  turgidity  in  the 
summer  of  1912,  but  no  measurable  growth;  in  the  spring  of  1913, 
after  the  winter  in  which  the  minimum  temperature  at  that  locality 
was  —6°,  the  plant  was  found  to  be  dead.  Although  the  rainfall 
at  Manzanita  Ridge  in  the  summer  of  1912  was  8.68  inches  as  com- 
pared with  5.61  inches  at  the  location  from  which  the  cactus  was 
taken  (near  the  3,000-foot  station),  it  was  not  able  to  seize  the 
advantage.  This  fact  itself  involves  the  factor  of  summer  temperature, 
which  doubtless  determines  the  rate  of  growth  of  the  roots  and  their 
power  for  the  intake  of  water. 

The  evidence  which  shows  Carnegiea  to  be  limited  in  its  upward 
distribution  by  the  greatest  number  of  freezing  hours  is  probably 
applicable  to  a  large  number  of  desert  plants,  non-succulent  as  well 
as  succulent  forms,  which  find  their  limitation  at  about  the  same 
elevation. 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  97 

THE  ROLE  OF  TOPOGRAPHIC  FEATURES  IN  DETERMINING  DEPARTURES 
FROM  THE  NORMAL  ALTITUDINAL  GRADIENT  OF  VEGETATION. 

The  normal  or  ideal  gradient  of  vegetation  is  disturbed  by  three 
sets  of  topographic  influences:  (a)  that  of  slope  exposure,  (b)  that  of 
the  surface  flow  or  underflow  of  streams  and  arroyos  and  the  high 
soil  moisture  of  flood-plains,  and  (c)  that  of  location  with  respect  to 
ridges,  slopes,  or  valley  bottoms,  which  may  be  designated  briefly  as 
the  influence  of  topographic  relief.  These  three  sets  of  topographic 
features  do  not  bring  into  operation  any  factors,  nor  any  intensities 
of  the  common  factors,  which  are  not  involved  in  the  normal  vertical 
gradients  of  physical  factors,  although  in  some  cases  they  bring  about 
new  combinations  of  factors  not  exactly  duplicated  at  other  elevations 
under  the  conditions  of  the  hypothetical  normal  gradient.  In  the 
description  of  the  vegetation  there  have  been  frequent  allusions  to 
these  three  sets  of  departures  from  the  ideal  gradient  of  vegetation. 
Instrumentation  has  also  been  described  which  throws  light  upon  the 
operation  of  slope  exposure  and  of  topographic  rehef.  The  influence 
of  streams  has  been  very  obvious  in  its  nature  and  has  not  been  investi- 
gated instrumentally. 

THE  ROLE  OF  SLOPE  EXPOSURE. 

The  importance  of  slope  exposure  in  determining  the  vertical  limits 
of  species,  and  in  thereby  determining  the  vertical  range  of  types  of 
vegetation,  has  been  a  matter  of  observation  and  comment  among 
almost  all  writers  on  the  vegetation  of  the  western  United  States. 
Although  the  phenomenon  is  of  universal  occurrence  throughout  the 
extra-tropical  portions  of  the  globe  it  is  rendered  particularly  striking 
in  regions  where  there  are  transitions  from  desert  or  grassland  into 
forested  country.  In  any  region  like  the  Santa  Catalina  Mountains, 
wdth  their  steep  climatic  gradient  and  varied  topography,  the  opera- 
tion of  the  factors  involved  in  slope  exposure  is  such  as  to  present  an 
alternation  of  vegetistic  regions,  causing  constant  departures  of  the 
vegetation  from  the  theoretical  norm  to  the  norm  of  higher  or  lower 
portions  of  the  mountain. 

Slope  exposure  is  a  "factor"  in  differentiating  the  vegetation  of 
opposed  slopes  at  all  elevations.  Even  at  altitudes  between  2,000  and 
3,000  feet  among  the  volcanic  hills  of  the  Tucson  region,  there  are 
conspicuous  differences  between  the  south  slopes,  with  their  heavy 
stands  of  Carnegiea,  Encelia  farinosa,  and  Opuntia  bigelovii,  and  the 
north  slopes  with  their  abundant  individuals  of  Parkinsonia  microphylla 
and  Lippia  wrightii  and  their  heavier  growth  of  perennial  grasses.* 
The  difference  between  northern  and  southern  exposures  is  most  con- 
spicuous between  4,000  and  5,000  feet,  where  the  former  have  orchard- 

*  See  Spalding,  V.  M.  Distribution  and  Movements  of  Desert  Plants.  Carnegie  Inst.,  Wash., 
Pub.  113,  1909. 


98  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

like  stands  of  evergreen  oaks  and  the  latter  are  treeless  (see  plate  9b)  . 
Almost  equally  striking,  however,  is  the  contrast  between  the  open 
pine  forests  on  south  slopes  at  9,000  feet  and  the  heavy,  deep-shaded 
stands  of  fir  on  north  slopes  at  the  same  elevation  (compare  plate 
29a  and  plate  35). 

Although  the  influences  of  slope  exposure  are  operative  at  all  eleva- 
tions they  acquire  added  power  with  increase  of  altitude.*  In  the 
Desert  region  and  the  Lower  Encinal  the  uppermost  limits  of  species 
on  north  slopes  and  on  south  slopes  are  from  600  to  1,000  feet  apart 
(Carnegiea,  Echinocactus,  Quercus  emoryi),  while  in  the  Forest  region 
the  upper  limits  on  opposed  slopes  differ  by  1,000  to  2,000  feet  {Quercus 
hypoleuca,  Juniperus  pachyphloea,  Arbutus  arizonica).  Another  test 
of  the  same  fact  may  be  had  by  comparing  a  north  slope  at  3,000  feet 
with  a  south  slope  at  6,000,  and  by  then  carrying  the  comparison  up 
3,000  feet.  Between  the  north  slope  at  3,000  feet  and  the  south  slope 
at  6,000  feet  is  the  strong  contrast  of  Desert  and  closed  Encinal,  with 
only  a  few  xerophilous  ferns  and  one  small  cactus  in  common.  Between 
the  north  slope  at  6,000  and  the  south  slope  at  9,000  feet  is  the  very 
close  resemblance  of  two  stands  of  Pine  Forest,  in  one  of  which  are  still 
to  be  seen  a  few  Encinal  forms  that  have  disappeared  from  the  other  and 
higher  one. 

The  increased  influence  of  slope  exposure  at  higher  elevations  is  not 
to  be  attributed  to  the  fact  that  the  species  of  the  Upper  Encinal  and 
Forest  range  through  greater  elevations  than  do  the  species  of  the 
Desert  and  the  Lower  Encinal.  The  number  of  feet  through  which 
a  species  ranges  on  south  slopes  or  on  north  slopes  has  no  necessary 
connection  with  the  difference  between  its  upper  limits  on  north  and 
on  south  slopes.  The  ability  of  a  large  number  of  plants  to  range 
through  a  greater  vertical  distance  in  the  Upper  Encinal  and  Forest 
than  it  is  possible  for  the  plants  of  the  lower  vegetations  to  do  may  be 
owing  to  the  ability  of  the  plants  of  the  upper  portion  of  the  mountain 
to  withstand  a  greater  gamut  of  conditions  than  the  plants  of  the  basal 
vegetations  can.  It  would,  in  any  event,  not  be  due  to  the  existence 
of  more  gradual  gradients  of  climatic  change  at  the  higher  elevations, 
since  in  every  case  of  the  measurement  of  these  gradients  they  have 
been  shown  to  grow  steeper  between  6,000  and  9,000  feet  than  below 
6,000  feet.  The  increase  in  the  effects  of  slope  exposure  with  increase 
of  altitude  can  only  be  ascribed  to  an  increasing  differentiation  of  the 
cHmatic  conditions  between  north  and  south  slopes  at  higher  elevations. 
An  examination  of  the  curves  of  evaporation  and  of  soil  moisture  (figs. 
10  and  14)  will  show  that  the  readings  for  the  highest  stations  exhibit 
the  greatest  apartness,  at  least  with  respect  to  the  intensities  involved. 

*  Merriam  has  illustrated  this  fact  in  a  diagrammatic  profile  of  San  Francisco  Peak,  but  has 
not  mentioned  it  in  the  text  of  his  paper.  See  Merriam,  C.  Hart.  Biological  Survey  of  the  San 
Francisco  Mountain  Region,  Arizona.    U.  S.  Dept.  Agric,  North  Amer.,  Fauna  No.  3,  1890,  pi.  1. 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  99 

It  is  obvious  that  the  importance  of  slope  exposure  Hes  in  the  topo- 
graphic control  of  the  physical  factors  which  form  the  environment  of 
the  plants  concerned.  It  is  possible  to  know,  on  purely  a  priori  grounds, 
that  two  slopes  of  the  same  inclination,  which  lie  in  opposed  positions 
so  that  one  faces  north  and  the  other  south,  will  present  to  plants  two 
environments  differing  in  almost  every  essential  physical  feature.  The 
temperature  of  the  air  on  two  such  slopes  might  be  identical  as  deter- 
mined by  the  thermometers  of  a  carefully  established  meteorological 
station,  but  they  are  distinctly  different  as  they  affect  the  vegetation, 
for  the  plants  not  only  receive  the  direct  rays  of  the  sun  but  receive 
very  different  amounts  of  heat  through  diurnal  terrestrial  radiation. 
This  circumstance  is  of  small  importance  to  full-grown  trees  and  large 
plants,  but  is  of  great  importance  to  young  plants  and  seedlings.  The 
soil  temperatures  of  opposed  slopes  are  also  widely  unlike,  even  in  the 
presence  of  the  undisturbed  cover  of  natural  vegetation.  The  two 
opposed  slopes  would  in  all  likelihood  receive  the  same  rainfall,  al- 
though this  is  not  necessarily  the  case.  An  equal  amount  of  rain  might 
effect  an  equal  elevation  of  the  soil  moisture  on  the  two  slopes,  and  to 
the  same  depth,  but  the  soil  evaporation  of  the  south  slope  would 
greatly  exceed  that  of  the  north  slope,  and  a  lower  moisture  would  soon 
prevail  in  the  soil  of  the  former.  Greater  or  less  differences  may  thus 
be  shown  to  obtain  between  the  opposed  slopes  with  respect  to  the 
most  vital  features  of  plant  environment.  Any  attempt  to  explain  the 
importance  of  slope  exposure  in  determining  plant  distribution  is  there- 
fore incomplete  unless  it  takes  into  account  every  possible  environ- 
mental difference  between  the  slopes.  Some  of  these  dift"erences  are 
undoubtedly  of  far  greater  importance  than  others,  but  the  question 
of  their  relative  importance  is  always  one  that  must  be  asked  with 
respect  to  a  particular  species  of  plant.  To  make  a  thoroughgoing 
answer  as  to  the  importance  of  slope  exposure  for  a  single  species  is  in 
itself  a  very  great  undertaking. 

The  universal  occurrence  of  a  large  number  of  species  of  plants  in 
the  vegetation  of  the  Santa  Catalinas,  their  commonness  within  their 
ranges,  and  the  consistency  of  their  distribution  with  respect  to  slope 
exposure,  all  indicate  that  there  has  been  ample  time  in  the  history  of 
the  mountain  for  all  of  these  species  to  attain  as  wide  a  distribution  as 
it  is  possible  for  them  to  have  under  existing  climatic  conditions.  It 
is  difficult  to  conceive  of  any  upward  or  downward  movement  being 
possible  for  any  of  the  common  species  of  plants,  inasmuch  as  thousands 
of  years  have  already  given  an  opportunity  for  such  extensions  of 
range.  In  view  of  the  steep  climatic  gradient  of  the  mountain  it  is 
easy  to  believe  that  all  of  the  common  species  have  reached  upper  and 
lower  limits  beyond  which  their  survival  is  prevented  by  definite  fea- 
tures of  the  physical  environment.  The  present  vertical  hmit  of  a 
species,  whether  upper  or  lower,  must  be  looked  upon  as  the  average 


100  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

point  at  which  some  particular  feature  of  its  physiological  activities 
is  met  by  some  particular  environmental  condition  that  is  preventive 
or  unduly  inhibitory  to  it.  The  minor  fluctuations  of  climate,  which 
have  their  minimal  and  maximal  values  within  periods  that  are  as 
brief  as  the  normal  Hfe  of  a  perennial  plant,  are  registered  in  the  infre- 
quency  of  every  species  as  it  approaches  its  distributional  limit  and 
in  the  scattered  individuals  which  lie  farthest  out  from  the  main  area 
of  occurrence.  The  secular  changes  of  climate  which  have  their  maxi- 
mal and  minimal  points  many  centuries  apart  are  registered  in  slight 
movements  of  the  limits  of  species,  the  marginal  region  of  scattered 
occurrence  being,  of  course,  the  first  affected  by  such  movements. 

The  writer  has  seen  no  evidence  indicating  that  competition  between 
plants  is  at  any  place  in  the  Santa  Catalinas  responsible  for  the  limi- 
tation of  any  species.  There  is,  of  course,  competition  such  as  that 
between  seedling  pines  in  heavy  stands  of  10  to  40  years  in  age,  and 
such  competition  as  occurs  between  individuals  of  the  same  or  different 
species  of  herbaceous  plants  in  small  areas  of  moist  flood-plain.  While 
competition  may  thus  determine  the  surviving  individuals  of  a  stand 
of  young  trees,  or  may  determine  the  composition  of  a  small  community 
of  ephemeral  or  root  perennial  plants,  it  is  not  responsible  for  the  find- 
ing of  a  plant  in  one  habitat  rather  than  in  another,  and  is  not  respon- 
sible for  the  exclusion  of  a  species  from  an  area  in  which  it  might  find 
favorable  conditions. 

It  is  only  consistent  with  our  knowledge  of  the  diversified  physical 
requirements  of  plants  that  there  should  be  such  great  diversity  in 
the  location  of  the  belts  of  altitude  occupied  by  different  species,  and 
it  accords  with  our  knowledge  of  the  distribution  of  plants  in  general 
that  these  belts  should  be  wider  in  some  cases  than  in  others.  It  is 
possible,  however,  to  pick  out  groups  of  plants  the  limits  of  which 
correspond  roughly  with  the  limits  of  the  Desert,  Encinal,  and  Forest 
types  of  vegetation  respectively.  Even  the  plants  of  equatorial  regions, 
in  which  there  is  a  notable  constancy  of  climate,  both  daily  and  annual, 
are  able  to  endure  small  ranges  of  climate,  or  occasionally  to  endure 
changes  in  individual  factors  which  are  many  times  as  great  as  the 
normal  fluctuations  of  their  native  climates.  In  addition  to  the 
fluctuations  of  climate  from  month  to  month  or  from  year  to  year  which 
must  be  endured  by  any  plant,  there  are  often  even  greater  differences 
which  must  be  simultaneously  endured  by  the  most  remotely  separated 
individuals  of  the  same  specific  stock.  For  example,  Asclepias  tuberosa 
is  found  from  Maine  to  Minnesota  in  the  north  and  from  Florida  to 
Texas  in  the  south,  and  thence  sporadically  in  the  mountains  westward 
to  Arizona.  In  view  of  the  prodigious  range  of  this  somewhat  poly- 
morphous plant  it  is  surprising  not  to  find  it  reaching  a  greater  elevation 
than  6,500  feet  in  the  Santa  Catalinas.  With  regard  to  possible  differ- 
ences in  the  physiological  behavior  of  the  most  remotely  separated 
individuals  of  a  plant  stock  of  such  wide  range,  we  know  little. 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  101 

To  find  a  plant  growing  only  on  a  north  slope  at  5,000  feet,  only  on 
a  south  slope  at  7,000  feet,  and  on  both  at  6,000  feet,  as  is  the  case  with 
Pinus  cembroides,  for  example,  means  that  there  is  much  in  common 
between  the  physical  conditions  on  the  north  slope  at  5,000  feet  and 
the  south  one  at  7,000.  If  such  reversals  of  habitat  in  relation  to  slope 
were  rare  it  would  only  be  warrantable  to  state  that  there  were  some 
physical  features  in  common  between  the  opposed  slopes,  but,  as 
already  stated,  there  are  only  two  common  plants  (aside  from  those 
of  the  streamways)  regarding  which  a  similar  statement  could  not  be 
made.  It  is  obvious,  therefore,  that  if  we  compare  separately  the  alti- 
tudinal  gradients  of  climatic  change  for  the  south  slopes  and  for  the 
north  slopes  of  these  mountains,  the  two  gradients  will  be  similar  in 
character  and  will  be  closely  related.  Their  relationship  will  consist 
in  the  fact  that  a  given  intensity  or  value  on  one  of  the  gradients  will 
be  found  on  the  other  at  a  lower  or  higher  elevation,  unless  barred  by 
the  base  or  summit  of  the  mountain.  In  the  curve  showing  the  alti- 
tudinal  gradients  of  evaporation  on  north  and  on  south  slopes  (fig.  14) 
it  will  be  seen  that  the  rate  on  the  south  slope  at  6,000  feet  is  exactly 
the  same  as  the  rate  on  the  north  slope  at  5,000  feet.  The  rate  on  the 
south  slope  at  8,000  feet,  however,  is  far  less  than  the  rate  on  the  north 
slope  at  7,000  feet.  The  latter  rate  is  found  on  south  slopes  at  about 
7,300  feet,  according  to  the  evidence  of  the  curve.  In  spite  of  differ- 
ences in  the  pitch  of  the  climatic  gradients  at  different  elevations,  it 
is  always  possible  to  find  a  slope  which  exhibits  the  same  intensity  of 
a  given  factor  as  that  which  has  already  been  found  on  an  opposed 
slope,  but  it  is  necessary  to  go  up  or  down  the  mountain  from  500  to 
1,500  feet  to  do  so.  It  might  be  possible  to  find  two  spots  on  opposed 
slopes  in  which  there  was  very  nearly  the  same  complex  of  all  environ- 
mental conditions,  although  the  finding  of  two  slopes  with  identical 
ones  would  be  rendered  almost  impossible  by  the  necessity  of  seeking 
these  spots  at  different  altitudes. 

Even  if  a  series  of  considerable  differences  were  found  between  the 
north  slope  at  5,000  feet  and  the  south  slope  at  7,000  feet,  on  both  of 
which  Pinus  cembroides  is  growing,  nevertheless  such  differences  would 
be  little  greater  than  those  which  are  met  by  this  species  as  it  grows 
on  both  north  and  south  slopes  at  6,000  feet  or  those  that  exist  between 
the  north  slopes  at  5,000  and  6,000,  or  the  south  slopes  at  6,000  and 
7,000  feet. 

The  physical  factors  which  underlie  the  influence  of  slope  exposure 
are  simply  a  special  case,  for  the  most  part,  of  the  same  factors  which 
cause  the  altitudinal  differentiation  of  the  vegetation  of  the  entire 
mountain.  The  only  instrumentation  carried  out  with  a  view  to  secur- 
ing a  measure  of  the  influence  of  slope  exposure  is  comprised  in  the  data 
on  soil  moisture  and  on  evaporation  for  north  and  south  exposures  at  7 
and  5  elevations  respectively  (see  tables  7  and  9  and  figs.  10, 13,  and  14) . 


102 


VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 


Neither  the  data  for  soil  moisture  nor  those  for  evaporation  show 
the  exact  alternation  exhibited  by  the  vegetation  itself,  by  virtue  of 
which  a  given  north  slope  is  similar  in  vegetation  to  a  south  slope 
about  1,000  feet  above  it,  and  a  given  south  slope  is  similar  to  a  north 
slope  about  1,000  feet  below  it  (with  the  exception  of  the  highest 
altitudes).  The  conditions  of  evaporation  found  through  the  range 
of  Pinus  cembroides,  which  has  been  used  as  an  example  of  the  effects 
of  slope  exposure,  are  indicated  in  figure  18,  where  curves  are  given 
showing  the  seasonal  march  of  evaporation  on  a  north  slope  at  5,000 
feet,  the  average  of  the  evaporation  on  north  and  south  slopes  at  6,000 
feet,  and  the  amounts  on  the  south  slope  at  7,000  feet.  These  curves 
follow  a  course  which  is  parallel  and  indicate  evaporation  conditions 
which  are  remarkably 
similar  for  the  lower,  cen- 
tral, and  upper  indi\dd- 
uals  of  this  pine,  except 
for  the  higher  rate  at 
5,000  feet  during  the  arid 
fore-summer. 

The  ratios  of  soil  mois- 
ture to  evaporation  at 
different  altitudes  have 
been  worked  out  sepa- 
rately for  the  north  and 
south  exposures  at  the  6 
stations  (see  table  20). 
Since  these  ratios  are  an 
expression  of  the  conditions  of  the  arid  fore-summer  they  must  be 
taken  as  elucidating  only  those  phases  of  slope  exposure  which  are 
themselves  due  to  the  climate  of  that  season.  Any  subsidiary  in- 
fluence of  temperature  in  affecting  the  slope  exposure  phenomena  of 
vegetation  in  the  Santa  Catalinas  still  awaits  a  full  investigation. 

The  comparative  conditions  of  the  lower,  central,  and  upper  habitats 
of  Pinus  cembroides  may  be  again  investigated  in  the  light  of  the  ratios, 
which  are  as  follows:  North  slope  at  5,000  feet  21.3,  average  of  north 
and  south  slopes  at  6,000  feet  24.5,  south  slope  at  7,000  feet  24.1.  These 
figures  indicate  a  still  more  remarkable  similarity  of  the  water  con- 
ditions in  the  three  habitats  than  the  evaporation  figures  do.  To 
compare  a  plant  of  lower  range  we  may  take  Agave  schottii,  which 
encounters  conditions  expressed  by  the  following  ratios :  north  slope  at 
4,000  feet  33.1,  average  of  north  and  south  slopes  at  5,000  feet  20.6, 
south  slope  at  6,000  feet  33.0.  These  figures  fail  to  show  as  close 
agreement,  but  indicate  a  close  similarity  of  the  water  conditions  at 
the  lowest  and  uppermost  habitats,  and  a  more  favorable  set  of  con- 
ditions in  the  central  habitat.    To  make  a  similar  comparison  for  a 


Fig.  18. — Graphs  showing  seasonal  march  of  rate  of  evap- 
oration for  a  north  slope  at  5,000  feet  (dotted  line),  the 
average  for  a  north  and  a  south  slope  at  6,000  feet  (solid 
line),  and  for  a  south  slope  at  7,000  feet  (broken  line). 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  103 

plant  of  higher  range  than  Pinus  cembroides  we  may  take  Quercus 
hypoleuca,  which  ranges  through  3,000  feet,  with  the  usual  alternation 
at  the  top  and  bottom  of  its  range.  The  ratios  for  its  habitats  are  as 
follows:  North  slope  at  6,000  feet  16.0,  average  of  north  and  south 
slopes  at  7,000  feet  16.6,  south  slope  at  8,000  feet  3.9.  Here  is  close 
agreement  of  the  ratios  for  the  lower  and  central  portions  of  the  range, 
with  a  much  lower  value  for  the  top,  indicating  that  in  spite  of  the 
ability  of  Qmrcus  hypoleuca  to  withstand  the  conditions  expressed  by 
the  ratio  of  16,  it  is  likewise  capable  of  withstanding  the  more  favorable 
water  conditions  indicated  by  the  ratio  of  3.9.  Here,  in  other  words, 
is  a  tj^ical  Encinal  plant,  accompanied  throughout  its  range  by  many 
others,  which  is  able  to  extend  up  to  an  elevation  at  which  the  water 
conditions  are  much  more  favorable  than  they  are  in  the  lower  part 
of  its  range.  This  is  a  thing  which  the  Desert  plants  do  not  do,  and 
the  reason  is  undoubtedly  that  the  plants  of  the  Desert  encounter 
unfavorable  temperature  conditions  at  the  same  elevations  at  which 
they  begin  to  encounter  more  favorable  water  conditions,  while  such 
a  plant  as  Quercus  hypoleuca  is  capable  of  withstanding  the  rigorous 
temperatures  of  8,000  feet  and  is  thereby  enabled  to  range  upward 
into  a  region  of  more  favorable  water  conditions. 

Allusion  has  been  made  to  the  more  pronounced  character  of  the 
effects  of  slope  exposure  at  higher  elevations.  It  is  of  interest  in  that 
connection  to  contrast  the  ratios  of  evaporation  to  soil  moisture  for 
similarly  located  pairs  of  habitats  at  low  and  at  high  altitudes.  For 
example,  the  north  slope  at  4,000  feet  has  a  ratio  of  33.1,  the  south 
slope  at  6,000  has  a  value  of  33.0.  To  carry  the  comparison  up  2,000 
feet:  the  north  slope  at  6,000  feet  has  a  ratio  of  16.0,  the  south  slope 
at  8,000  feet  has  one  of  3.9.  The  greater  similarity  of  the  ratios  for 
the  two  lower  habitats  is  in  accord  with  the  evidences  from  the  vege- 
tation (see  p.  98),  which  indicate  an  altitudinal  increase  in  the  potency 
of  slope  exposure  in  the  determining  of  the  vegetation. 

The  fundamental  causes  differentiating  the  conditions  on  opposed 
slopes  are  only  partly  comprised  in  the  evaporation — soil-moisture  ratios. 
The  differences  of  evaporation  rate  on  north  and  south  slopes  are 
largely  due  to  the  dry,  warm  winds  which  ascend  the  mountain  during 
the  day  and  partly  to  the  differences  of  air  temperature.  The  humidity 
of  the  air  shows  only  slight  differences  on  opposed  slopes.  The  soil 
moisture  on  north  slopes  is  higher  than  on  south  ones  because  of  the 
more  direct  insolation  on  south  slopes,  and  because  of  the  higher  soil 
temperature  and  increased  soil  evaporation  which  are  due  to  this.  In 
addition  to  the  differentiating  features  which  are  expressed  in  the 
ratios,  we  have  the  differences  of  soil  temperature,  due  to  the  direction 
of  slope,  and  the  differences  of  air  temperature,  which  are  only  partially 
registered  in  their  effect  upon  the  evaporation  rate.  The  increased 
insolation  on  slopes  as  compared  with  level  ground  has  been  worked 


104  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

out  by  Hall,*  who  shows  that  the  amount  of  radiant  energy  reaching 
a  south  slope  of  45°,  with  the  sun  45°  above  the  horizon,  is  1.4  times 
as  great  as  that  reaching  a  level  piece  of  ground  through  an  aperture 
of  the  same  size.  It  is  through  this  difference,  which  is  still  greater 
between  south  and  north  slopes,  that  the  soil  is  given  a  higher  tempera- 
ture, that  the  air  is  warmed  to  a  higher  degree  through  radiation,  and 
the  soil  dried  more  rapidly  on  all  south-facing  exposures. 

THE  ROLE  OF  STREAMS  AND  FLOOD-PLAINS. 

In  the  description  of  the  vegetation  of  the  Santa  Catalinas  constant 
allusion  has  been  made  to  the  distinctive  plant  communities  of  springs, 
streams,  flood-plains,  and  arroyos.  The  contrast  between  the  vege- 
tation of  these  moist  or  relatively  moist  situations  and  that  of  the 
mountain  slopes  is  very  striking  at  the  mouths  of  the  larger  canons, 
and  throughout  the  Desert  and  Encinal  regions.  At  the  higher  alti- 
tudes, and  particularly  in  the  Fir  Forest,  the  moist  habitats  are  not 
only  less  striking  to  the  casual  observer,  but  their  vegetation  actually 
comprises  a  great  many  species  which  are  frequently  found  away  from 
proximity  to  streams. 

The  influence  of  streams  and  flood-plains  consists,  in  brief,  in  bring- 
ing components  of  the  upland  vegetation  of  each  altitude  down  along 
the  streamways  of  the  altitudes  just  below.  In  this  manner  the  Encinal 
is  traversed  by  bands  of  Forest,  and  the  Desert  slopes  are  traversed 
by  bands  of  Encinal.  Furthermore,  the  streams  and  springs  of  the 
mountain  afford  the  sole  habitats  for  a  number  of  species  of  aquatic 
and  palustrine  plants  which  do  not  appear  on  the  upland  at  any 
elevation. 

The  mechanical  agencies  of  gravity,  sheet-floods,  and  stream  flow 
are  all  capable  of  aiding  in  the  downward  dissemination  of  the  seeds 
of  all  mountain  plants  and  these  mechanical  agencies  should  assure 
the  occurrence  of  all  mountain  plants  in  all  situations  at  lower  altitudes 
in  which  they  are  capable  of  survival.  The  number  of  seeds  which 
are  borne  down  by  streams  is,  of  course,  enormous,  and  the  number  of 
resulting  germinations  is  probably  very  large.  The  number  of  sur- 
vivals, however,  is  controlled  by  the  physical  conditions  of  the  new 
low-altitude  habitat,  and  in  a  manner  to  be  further  considered. 

In  the  discussion  of  slope  exposure  no  account  has  been  taken  of  the 
occurrence  of  plants  along  streamways  at  elevations  below  their  lowest 
upland  occurrence,  since  the  individuals  along  the  arroyos  and  streams 
are  subjected  to  a  very  different  set  of  environic  controls  from  those 
that  determine  the  location  of  the  upland  individuals.  In  the  earlier 
discussion  of  the  vertical  limits  of  species  the  streamway  occurrences 
were  taken  into  account. 

*  Hall,  H.  M.  A  Botanical  Survey  of  San  Jacinto  Mountain.  Univ.  Cal.  Pubn.  Bot.,  vol.  i, 
pp.  1-140,  1902. 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  105 

Among  the  palustrine  plants  which  occur  along  streams  at  7,000  to 
8,000  feet  are  two  species  of  Juncus  and  two  of  Carex  which  also  occur 
in  Sabino  Canon,  under  the  most  favorable  conditions  of  moisture 
supply,  at  3,000  to  3,200  feet  elevation.  The  perennial  composite 
Tagetes  lemmoni  grows  along  the  drier  arroyos  of  the  Pine  Forest  down 
to  6,000  feet,  and  is  found  in  lower  Sabino  Cafion  growing  along  the 
margin  of  the  stream  at  3,200  feet.  Other  palustrine  plants  of  the 
Forest  region  are  found  from  time  to  time  at  low  elevations  along  the 
largest  streams,  but  no  others  have  been  observed  to  become  thoroughly 
established  there. 

The  well-known  cosmopolitanism  of  many  aquatic  plants  would 
cause  us  to  expect  such  behavior  as  is  exhibited  by  Juncus,  Carex,  and 
Tagetes  in  the  Santa  CataHnas.  There  are  several  species  of  Scirpu^ 
and  Eryngium,  and  at  least  one  woody  plant  {Cephalanthus  occidentalis) 
which  range  from  the  Gulf  of  Mexico  across  the  southwestern  boundary 
of  the  United  States  to  Cahfornia.  The  individuals  of  these  species 
are  subjected  to  a  wide  diversity  of  atmospheric  humidities,  but  are 
all  found  under  conditions  of  closely  equivalent  high  soil  moisture. 

A  greater  interest  attaches,  in  the  present  connection,  to  the  cases 
of  low  streamside  occurrence  of  plants  which  grow  typically  in  upland 
situations.  Mention  has  already  been  made  of  the  trees  of  Quercus 
arizonica  and  Quercus  oblongijolia  which  grow  along  the  Sabino  Creek 
at  2,800  feet,  about  1,200  feet  below  their  lowest  occurrence  on  north 
slopes.  Small  plants  of  Quercus  hypoleuca  have  been  found  growing  in 
deep  shade  in  the  bed  of  Sabino  Cafion  at  3,200  feet,  which  is  2,700 
feet  below  the  lowest  north  slope  occurrence  of  this  tree.  The  first- 
named  oaks  have  descended  no  further  than  many  other  upland  plants 
have  done,  but  the  last-named  oak  shows  the  most  pronounced  de- 
pression of  range  that  has  been  detected. 

At  the  mouth  of  Soldier  Cafion,  at  3,000  feet,  the  writer  has  found 
one  or  two  individuals  each  of  Dasylirion  ivheeleri.  Mimosa  biuncifera, 
Erythrina  coralloides,  and  Asclepias  linifolia.  At  an  elevation  of  4,500 
feet  Dasylirion  and  Asclepias  have  begun  to  appear  on  slopes  of  south 
exposure,  and  at  5,000  feet  Erythrina  and  Mimosa  have  also  left  the 
arroyos. 

At  4,900  feet  Ceanothus  fendleri  is  found  in  the  shade  of  oaks  on  the 
flood-plain  of  Soldier  Cafion.  It  occurs  also  at  5,300  feet  in  similar 
situations  at  the  head  of  Soldier  Canon,  and  becomes  frequent  in  the 
Upper  Encinal  at  6,000  feet.  Similarly  Quercus  suhmollis  occurs  near 
the  constant  water  at  Horse  Camp,  in  Bear  Canon,  at  6,100  feet  and 
is  of  increasing  frequence  along  streams  up  to  7,200  feet.  At  that 
elevation  and  up  to  the  uppermost  limit  of  Pine  Forest  it  is  conunon 
on  slopes  as  well  as  near  streams.  Robinia  neomexicana  is  found  in 
the  flood-plain  of  Soldier  Canon,  near  a  spring,  at  5,300  feet,  and  first 
becomes  a  frequent  upland  shrub  of  the  Pine  Forest  at  about  7,500  feet. 


106  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

It  is  possible  to  say,  in  brief,  that  the  conditions  presented  by  stream- 
sides  and  flood-plains  are  such  as  to  depress  the  ranges  of  very  many 
plants  by  as  much  as  1,000  feet,  and  of  a  few  plants  by  amounts  as 
great  as  2,000  feet.  A  depression  of  as  much  as  2,700  feet,  found  in 
the  case  of  Quercus  hypoleuca,  does  not  represent  the  lowest  occurrence 
of  established  plants,  but  rather  a  chance  survival  at  an  elevation  in 
which  it  would  doubtless  be  impossible  for  the  tree  to  reach  maturity. 
It  can  at  least  be  said  that  tliroughout  the  entire  length  of  Sabino 
Caiion,  from  the  mouth  to  the  Basin,  there  are  no  known  occurrences 
of  full-grown  trees  or  even  shrubs  of  Quercus  hypoleuca. 

The  extent  to  which  the  types  of  vegetation  are  depressed  in  their 
ranges  by  the  influence  of  streams  and  flood-plains  is  about  the  same 
as  the  average  depression  of  the  individual  species,  that  is  to  say  about 
1,000  feet.  In  the  case  of  the  occurrence  of  a  closed  Encinal  in  the 
Basin  of  Sabino  Canon  there  has  been  a  depression  of  1,500  feet  in 
the  limit  of  this  type  of  vegetation — from  5,500  to  4,000  feet. 

Some  evidence  has  already  been  given  leading  to  the  view  that  the 
lower  limits  of  all  Encinal  and  Forest  plants  are  determined  by  those 
features  of  the  environment  which  in  turn  determine  the  water  relation 
of  plants.  The  facts  of  the  depression  of  vertical  ranges  by  streams 
form  an  additional  evidence  of  this  view.  So  far  as  concerns  atmos- 
pheric water-demand  the  plants  growing  beside  streams  are  subjected 
to  the  same  conditions  as  plants  of  the  nearby  upland,  but  the  conditions 
of  water  supply  are  infinitely  better  for  them.    In  other  words,  in  the 

E 

ratio  of  evaporation  to  soil  moisture,  ^^,  the  numerator  is  the  same 
for  stream-side  and  upland  plants  and  the  denominator  is  greatly 
increased  for  the  latter,  thereby  lowering  the  values  for  the  ratio.  In 
the  cases  alluded  to  in  which  the  lowest  individuals  of  a  species  not 
only  grow  in  a  flood-plain  but  in  the  shade  of  larger  vegetation,  the 
plants  are  under  ameliorated  conditions  with  respect  to  the  numerator 
as  well  as  the  denominator  in  the  ratio. 

A  number  of  mountain  plants  are  able  to  survive  when  taken  down 
to  the  Desert  provided  they  are  placed  under  conditions  in  which  one 
or  both  of  the  sets  of  conditions  indicated  by  the  above-mentioned 
ratio  are  ameliorated.  Parthenocissus  from  6,000  feet  survives  with 
irrigation  and  partial  shade;  Echinocereus  polyacanthos,  from  5,000  to 
7,000  feet,  survives  with  occasional  irrigations  during  the  arid  fore- 
summer;  Zauschneria  calif ornica,  Aquilegia  chrysantha,  and  Sedum  stelli- 
forme,  all  ranging  from  5,500  to  7,500  feet,  are  capable  of  survival  at 
Tucson  from  year  to  year  when  grown  in  complete  shade  with  frequent 
irrigation  during  the  arid  fore-summer.  These  facts  point  to  the  ability 
of  such  plants  to  withstand  at  least  the  shade  temperatures  of  the 
Desert,  provided  the  moisture  supply  of  the  soil  and  the  moisture 
requirement  of  the  air  are  made  more  nearly  like  those  conditions  in 
the  mountain  habitats  of  the  plants.    Many  introduced  plants  have 


CORRELATION  OF  VEGETATION  AND  CLIMATE.  107 

shown  themselves  incapable  of  withstanding  the  atmospheric  aridity 
at  Tucson  even  when  grown  under  the  most  liberal  irrigation.  The 
inability  of  a  plant  to  pass  water  on  to  its  transpiring  tissues  as  rapidly 
as  it  is  withdrawn  by  a  desert  atmosphere  is  undoubtedly  a  feature 
common  to  very  many  mesophilous  plants,  and  it  is  apparently  the 
cause  which  prevents  a  greater  number  of  palustrine  mountain  plants 
from  descending  the  large  streamways  to  the  Desert,  and  it  doubtless 
prevents  a  lower  descent  upon  the  part  of  many  Forest  species  which 
reach  the  flood-plains  of  the  Lower  Encinal. 

It  might  be  argued  that  the  low  occurrence  of  Forest  along  the 
streams  of  the  Encinal  and  the  descent  of  the  Encinal  into  the  Desert 
Slopes  are  due  to  the  influence  of  cold-air  drainage  rather  than  to  the 
effects  of  soil  moisture,  or  that  cold-air  drainage  is  at  least  an  important 
contributory  factor.  It  is  difficult  to  believe  that  low  temperatures, 
especially  those  of  the  winter  months,  should  be  a  favoring  factor  for 
plants  which  are  subjected  during  the  day  to  just  as  high  temperatures  as 
are  the  plants  of  the  upland.  During  the  summer  months  the  low  noctur- 
nal temperatures  might  be  of  some  slight  importance,  but  such  importance 
would  reside  solely  in  aiding  the  plant  to  recover  from  the  excessive 
transpiration  of  the  preceding  day  and  to  build  up  a  reserve  of  water 
against  the  transpiration  of  the  following  day,  as  has  been  shown  by 
Edith  B.  Shreve  to  occur  in  Parkinsonia  microphylla*  The  facts  that 
it  is  the  highest  diurnal  temperatures  that  are  apt  to  be  deleterious 
to  low-ranging  mountain  plants  and  that  their  effect  can  be  only 
indirectly  and  slightly  offset  by  the  lowest  nocturnal  temperatures 
make  it  appear  that  cold-air  drainage  has  at  least  a  very  minor  role 
in  this  connection  as  compared  with  the  moisture  conditions. 

THE  ROLE  OF  TOPOGRAPHIC  RELIEF. 

Each  of  the  leading  types  of  vegetation  in  the  Santa  Catalinas 
reaches  the  uppermost  limit  of  its  occurrence  on  ridges  and  high  south- 
facing  slopes.  This  carries  the  Desert  upward  into  the  Encinal  and 
carries  the  Encinal  up  into  the  Forest  in  such  a  manner  that  there  is 
an  interdigitation  of  the  vegetistic  regions  throughout  the  portions  of 
the  mountain  in  which  the  topography  is  mature  enough  for  it  to  be 
manifest.  This  appearance  of  interdigitation  is  partly  brought  about  by 
the  influence  of  streams  (which  has  just  been  discussed)  and  is  some- 
times merged  with  the  influence  of  slope  exposure.  These  facts  do  not 
in  the  least  obscure  the  high  range  of  each  type  of  vegetation  on  the 
narrow  ridges  which  point  due  south  or  north  and  are  therefore  free 
from  the  influence  of  slope  exposure. 

On  the  ridges  which  lie  between  the  tributaries  of  Soldier  Canon 
have  been  found  the  highest  individuals  of  all  of  the  characteristic 

*  Shreve,  Edith  B.  The  Daily  March  of  Transpiration  in  a  Desert  Perennial.  Carnegie  Inst. 
Wash.,  Pub.  194,  1914. 


108  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

species  of  the  Desert  (for  elevations  see  page  37).  On  a  high  ridge 
tributary  to  Bear  Canon  have  been  found  the  highest  individuals  of 
Opuntia  sp.,  the  highest  species  of  that  genus  on  the  mountain,  and 
Mamillaria  grahami,  the  highest-ranging  plant  of  the  Desert.  The 
individuals  which  most  nearly  approach  these  highest  stations  for 
Opuntia  and  Mamillaria  have  been  found  on  south  exposures  about 
600  feet  lower,  in  the  Bear  Canon  drainage. 

On  an  exposed  ridge,  with  a  considerable  inclination  to  the  south, 
at  7,800  feet  are  found  the  highest  individuals  of  Pinus  cembroides, 
Juniperus  pachyphloea  (with  one  known  exception).  Yucca  schottii, 
Echinocereus  poly  acanthus,  and  Arctostaphylos  pungens.  In  this  station 
the  influences  of  slope  exposure  and  of  topographic  relief  are  combined, 
thereby  bringing  about  the  pronounced  conditions  that  are  expressed 
in  the  highest  occurrence  of  5  species  of  the  Upper  Encinal.  On  the 
ridges  above  Marshall  Gulch  are  found  the  highest  occurrences  of 
Quercus  hypoleuca  and  Quercus  reticulata,  both  of  which  forms  extend 
further  down  the  south  faces  of  these  east-and-west  ridges  than  they 
do  down  the  north  faces. 

When  Desert  plants  are  found  on  the  ridges  of  the  Encinal  region 
they  fail  to  appear  on  the  south-facing  slopes  just  below  these  ridges. 
When  the  plants  of  the  Encinal  are  found  at  their  highest  locations 
on  ridges  of  the  Forest  Region  they  are  also  absent  on  the  south-facing 
slopes  just  below  the  ridges.  This  does  not  appear  to  be  the  case  with 
respect  to  the  highest  occurrences  of  plants  which  are  believed  to  have 
their  true  climatic  limit  just  below  the  sunamit  of  Mount  Lemmon, 
such  as  Quercus  hypoleuca  and  Quercus  reticulata. 

The  extent  by  which  the  highest  individuals  on  ridges  exceed  the 
highest  individuals  on  south  slopes  is  never  more  than  500  to  600  feet, 
except  in  the  case  of  Pinu^  cembroides,  in  which  it  is  about  700  feet. 
Opuntia  sp.  and  Mamillaria  grahami,  which  have  their  upper  limit  in 
the  vicinity  of  7,000  feet,  agree  in  this  respect  with  Opuntia  versicolor, 
Echinocactus  wislizeni,  and  Fouquieria  splendens,  which  have  their 
limit  in  the  vicinity  of  5,500  feet. 

Perhaps  the  most  common  explanation  of  the  highest  occurrence  of 
species  on  ridges  is  that  the  soil  is  driest  in  such  situations  and  therefore 
offers  to  plants  from  lower  elevations  a  habitat  more  like  that  in  which 
they  are  abundant.  The  principal  objection  to  such  an  explanation 
is  the  unquestionable  fact  that  a  somewhat  more  moist  soil  is  not 
inimical  to  the  plants  of  the  Desert  nor  to  the  plants  of  the  Encinal. 
Neither  is  there  a  sufficient  difference  between  the  soil  moisture  at 
the  bottom  of  a  slope  and  on  the  ridge  at  the  top  of  the  slope,  in  the 
arid  seasons,  to  cause  a  differentiation  of  the  vegetation. 

The  explanation  of  the  phenomenon  may  be  sought  partly  in  the 
existence  of  cold-air  drainage,  which  is  at  least  responsible  for  the 
absence  of  Desert  and  Encinal  plants  from  the  bottoms  of  canons  at 


GENERAL  CONCLUSIONS.  109 

the  highest  elevations  to  which  they  attain.  The  streams  of  cold  air 
are  not  more  than  75  to  100  feet  deep,  however,  and  can  not,  therefore 
be  functional  in  preventing  the  occurrence  of  plants  on  the  middle 
and  upper  slopes  of  canons.  An  apparently  valid  explanation  of  the 
high  occurrences  on  ridges  is  in  accordance  mth  the  theory  already 
mentioned,  that  the  upper  limits  of  the  Desert  species,  and  possibly 
of  the  Encinal  species  also,  are  set  by  winter  temperature  conditions. 
The  ridges  are  obviously  the  localities  which  receive  the  fullest  and 
longest  insolation  on  the  short  winter  days  with  low  sun.  This  cir- 
cumstance would  not  only  warm  the  plants  themselves  but  would 
warm  the  soil  and  rocks  in  a  manner  such  as  to  lessen  the  severity  of 
the  coldest  nights.  With  the  pronounced  low  temperatures  in  the 
canons,  due  to  cold-air  drainage,  and  with  the  favorable  conditions 
of  the  ridges  for  a  pre-warming  of  both  plant  and  habitat,  it  may  be 
expected  that  there  will  be  great  differences  between  the  vertical 
limits  of  species  in  canon  bottoms  and  on  ridges. 

GENERAL  CONCLUSIONS. 

The  desert  mountain  ranges  of  the  southwestern  United  States  stand 
in  the  midst  of  a  region  which  presents  severe  conditions  for  plants. 
The  relative  richness  of  the  vegetation  in  this  region  is  due  chiefly  to 
the  occurrence  of  two  yearly  seasons  of  rainfall.  The  entire  annual 
vegetational  behavior  is  related  primarily  to  the  moisture  seasons  and 
much  less  pronouncedly  to  the  thermal  seasons.  The  perennial  plants 
lead  an  existence  which  permits  of  rapid  growth  during  the  warm 
humid  season,  together  with  an  extremely  low  ebb  of  activity  during 
the  arid  seasons,  and  with  the  possible  loss  through  drought-death  of 
much  of  the  growth  that  has  just  taken  place. 

The  severe  conditions  of  the  desert  environment  cause  the  vegetation 
to  exhibit  a  high  degree  of  sensitiveness  to  slight  topographic  and 
edaphic  differences.  Wherever  the  character  of  the  soil  or  the  topo- 
graphic location  is  such  as  to  present  a  degree  of  soil  moisture  slightly 
above  that  of  the  general  surroundings,  or  as  to  maintain  it  for  a  longer 
time  in  the  periods  of  extreme  aridity;  or  in  whatever  locations  plants 
are  protected  from  the  most  extreme  conditions  of  transpiration — in 
such  places  are  to  be  found  heavier  stands  of  vegetation  or  else  particular 
species  of  plants. 

The  higher  mountains  of  the  desert  region  exhibit  strong  gradients 
of  change  in  climate  and  in  vegetation.  Both  of  these  gradients  are 
much  more  pronounced  than  those  of  mountains  of  equal  elevation 
in  more  humid  regions.  They  lead  from  arid  to  humid,  or  at  least 
semi-humid,  conditions  of  moisture,  and  from  sub-tropical  to  tem- 
perate conditions  of  temperature;  from  low,  open  microphyllous  and 
succulent  desert,  through  a  sclerophyllous  semi-forest  to  heavy  conif- 
erous forest. 


110  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

The  sensitiveness  which  desert  vegetation  exhibits  to  sUght  environ- 
mental differences  is  even  more  pronounced  with  respect  to  the  climatic 
gradients  of  the  mountains.  Throughout  a  vertical  range  of  6,000 
feet  there  is  not  only  a  very  striking  gradient  of  vegetation,  but  a 
very  nice  adjustment  of  vegetation  to  the  physical  conditions.  In  the 
Desert  and  Encinal  regions,  and  to  a  great  extent  in  the  Forest  as  well, 
this  is  chiefly  an  adjustment  of  plant  to  environment  and  scarcely  at 
all  an  adjustment  of  plant  to  plant.  Every  juvenile  individual  in  the 
open  Desert  and  Encinal  regions  is  a  pioneer,  and  on  reaching  maturity 
this  individual  is  part  of  an  ultimate  stable  community. 

The  principal  features  of  altitudinal  climatic  change  are :  the  short- 
ening of  the  frostless  season,  the  lowering  of  the  daily  curve  of  tempera- 
ture throughout  the  frostless  season,  the  increasing  of  the  intensity 
and  duration  of  all  critical  phases  of  low  temperature  during  the  frost 
season,  the  shortening  of  the  arid  fore-summer  (the  critical  season  of 
aridity),  the  increasing  of  precipitation  and  therefore  of  soil  moisture, 
and  the  decreasing  of  evaporation. 

On  a  mountain  having  the  form  of  a  smooth  cone  it  would  be  possible 
to  observe  the  ideal  manner  in  which  these  climatic  gradients  would 
collectively  control  the  vertical  distribution  of  the  vegetation.  The 
occurrence  in  nature  of  irregularities  of  relief  is  responsible,  however, 
for  local  departures  from  the  ideal  vertical  gradients  of  climate  and 
also  from  the  ideal  altitudinal  distribution  of  vegetation  which  would 
be  anticipated  on  a  geometrically  constructed  mountain.  It  is  possible, 
nevertheless,  to  correlate  the  climatic  and  vegetational  gradients  in 
spite  of  the  local  irregularities  of  each  of  them,  and  in  fact  the  study 
of  these  departures  from  the  ideal  has  aided  in  the  interpretation  of 
the  correlations. 

The  vertical  distribution  of  vegetation  on  the  Santa  Catalina  Moun- 
tains has  been  found  to  be  due  to  the  interaction  of  two  sets  of  controls 
which  are  nearly  distinct.  One  of  these  controls  has  its  seat  in  the 
moisture  conditions,  the  other  in  the  temperature  conditions.  The 
temperature  control  has  been  studied  experimentally  only  with  respect 
to  three  species  of  plants,  but  it  is  believed  on  this  evidence  (as  well 
as  the  evidence  of  the  departures  from  the  normal  gradient  of  vegeta- 
tion, correlated  with  instrumentation)  to  be  the  control  which  limits 
the  upward  distribution  of  the  Desert  species  and  perhaps  of  some 
species  of  the  Encinal.  The  moisture  control  has  not  been  studied 
experimentally  in  connection  with  the  present  investigation,  but  its 
operation  is  well  known,  and  the  instrumental  study  of  soil  moisture 
and  evaporation  at  successive  altitudes,  with  due  attention  to  the 
departures  from  the  normal  gradient  of  vegetation,  has  indicated  that 
the  ratio  of  the  latter  factor  to  the  former  affords  a  concise  expression 
of  the  control  which  limits  the  downward  distribution  of  Forest  and 
Encinal  plants. 


GENERAL  CONCLUSIONS.  Ill 

The  principal  departures  of  the  vegetation  from  the  ideal  gradient 
that  would  be  found  on  a  geometrical  cone  are  expressed  in  the  irregu- 
larity of  the  upper  or  lower  limits  of  vegetations  or  of  individual  species 
as  observed  in  different  habitats.  The  chief  departure  is  that  due  to 
slope  exposure,  by  virtue  of  which  the  vegetation  of  north-facing  and 
south-facing  slopes  at  the  same  elevation  shows  striking  differences. 
A  second  departure  is  that  due  to  the  influence  of  streams  and  the 
high  moisture  content  of  the  soil  of  arroyos  and  flood-plains,  by  reason 
of  which  the  plants  of  all  altitudes  are  carried  below  their  normal 
lowest  occurrences  on  slopes.  Another  departure  is  due  to  the  influence 
of  ridges,  on  which  the  plants  of  all  elevations  (and  particularly  those 
of  the  Desert)  find  their  highest  occurrences.  These  departures  seldom 
result  in  the  occurrence  of  distinctive  plant  communities,  but  are 
operative  rather  in  the  carrying  of  the  usual  and  widespread  communi- 
ties into  elevations  at  which  they  are  exceptional.  The  effect  of  slope 
exposure  is  to  carry  the  normal  vegetation  of  a  given  elevation  both 
up  and  down  the  mountain,  so  that  its  lowest  occurrences  are  on  north 
slopes  and  its  highest  on  south  slopes.  The  effect  of  streamways  is 
to  carry  either  the  normal  or  the  streamside  vegetation  down  the  moun- 
tain, so  that  the  extreme  lowest  occurrences  of  almost  all  Encinal  and 
Forest  plants  may  be  sought  along  the  streamways.  The  effect  of 
ridges  is  to  carry  the  vegetation  (or  more  particularly  individual 
species  and  small  groups  of  species)  up  the  mountain,  so  that  all  highest 
occurrences  of  Desert  and  Encinal  species  are  to  be  found  on  narrow 
ridges — the  highest  occurrences  of  Forest  plants  are  not  reached  on 
the  Santa  Catalina  Mountains,  and  they  are  controlled  by  a  very 
dissimilar  group  of  factors. 

It  is  impossible  to  study  the  distribution  of  vegetation  in  a  region 
where  pronounced  differences  may  be  found  within  short  distances 
without  being  impressed  with  the  independence  which  each  species 
exhibits  in  its  allocation.  Plants  which  are  associated  on  the  Lower 
Desert  Slopes,  for  example,  range  to  very  different  maximum  altitudes, 
and  plants  which  are  associated  in  the  Upper  Encinal  are  found  to  be 
in  part  at  the  upper  edges  of  their  ranges,  in  part  at  the  lower  edges, 
and  also  in  part  rather  closely  restricted  to  that  region.  It  is  nowhere 
possible  to  pick  out  a  group  of  plants  which  may  be  thought  of  as 
associates  without  being  able  to  find  other  localities  in  which  the  asso- 
ciation has  been  dissolved.  Certain  plants  may  be  thought  of  as  having 
closely  identical  physical  requirements  because  of  their  associated 
occurrence  in  the  same  spot.  Nevertheless,  the  fact  that  the  vertical 
ranges  and  habitat  characteristics  of  these  species  will  reveal  more 
or  less  pronounced  differences  goes  to  show  that  each  of  them  has 
survived  in  a  particular  section  of  the  climatic  gradient.  It  is  true  in 
the  Santa  Catalina  Mountains,  as  it  is  true  in  all  other  places,  that  the 
associated  members  of  a  plant  community  are  not  able  to  follow  each 


112  VEGETATION  OF  A  DESERT  MOUNTAIN  RANGE. 

other  to  a  common  geographical  and  habital  Umit.  The  physical 
requirements  of  plants  are  so  varied  and  so  elastic  that  the  composition 
of  a  series  of  communities  occupying  similar  habitats  in  widely  sepa- 
rated places  shows  the  constant  overlapping  of  the  ranges  of  individual 
species  which  is  due  to  the  physiological  inequivalence  of  these  species. 

It  is  particularly  true  of  the  plant  communities  of  arid  and  semi- 
arid  regions  that  the  most  closely  associated  individuals  are  not  alike 
in  their  life  requirements,  and  this  is  true  to  a  less  pronounced  extent 
in  all  plant  communities.  The  members  of  the  many  diverse  biological 
types,  or  growth  forms,  which  are  found  together  in  Desert  and  Encinal 
find  their  soil  water  at  different  levels,  procure  it  at  different  seasons, 
and  lose  it  through  dissimilar  fohar  organs,  at  the  same  time  that  they 
react  differently  to  the  same  temperature  conditions.  In  brief,  these 
associated  plants  are  not  living  in  the  same  climate  but  are  living  in 
different  sections  of  the  same  climate,  the  demarcation  of  these  sections 
being  either  temporal  or  spatial. 

The  use  of  the  physical  characteristics  of  the  habitat  as  a  criterion 
in  the  definition  of  a  plant  community  does  something  to  give  a  greater 
rigidity  and  a  wider  applicability  to  the  definition.  On  the  other  hand 
it  confuses  cause  with  effect  and  makes  it  impossible  to  investigate 
the  relation  of  physical  conditions  to  a  community  defined  in  that 
manner  without  reopening  the  whole  question  as  to  the  nature  and 
identity  of  the  community.  There  is  much  strong  logic  to  support 
the  view  that  all  necessary  definitions  and  classifications  of  vegetation 
should  be  made  on  the  basis  of  the  vegetation  alone.  When  units  of 
vegetation  are  thus  defined  they  lend  themselves  to  the  further  study 
of  their  life  requirements,  and  it  is  such  study — applied  to  individual 
species  as  well  as  to  vegetation — that  affords  the  most  promising  and 
important  field  for  ecological  activity. 

The  distribution  of  vegetation  in  the  Santa  CataHna  Mountains  is 
strongly  controlled  by  a  steep  climatic  gradient;  the  vegetation  itself 
is  diversified  in  its  display  of  growth  forms;  and  the  secular  changes 
of  vegetation  due  to  physiographic  phenomena,  and  to  the  reaction 
of  the  plant  upon  its  habitat,  are  in  almost  complete  abeyance.  These 
circumstances  have  made  it  possible  to  give  a  delineation  of  the  vege- 
tation upon  purely  vegetational  characteristics,  without  regard  to  the 
secular  changes  which  are  taking  place  in  very  restricted  areas,  and 
with  particular  emphasis  upon  the  individualism  of  behavior  among  the 
characteristic  species.  The  same  circumstances  have  also  made  it 
possible  to  lay  side  by  side  the  facts  respecting  the  vegetational  gradient 
and  those  respecting  the  climatic  gradient  in  such  manner  as  to  reveal 
the  correlations  between  the  two  and  to  indicate  some  of  the  physical 
controls  which  operate  in  the  limitation  of  the  activities  and  of  the 
ranges  of  species  and  of  vegetations. 


SHREVE 


Plate  1 


SHREVE 


Plate  2 


A.  South  face  of  Santa  Catalina  Mountains  viewed  7  miles  from  their  base.    Mount  Lemmuu  is  un  riglit  center. 
In  foreground  is  bajada  vegetation  of  Covillea  tridentata,  Opuntia  spinosior,  and  Isocotna  hartwegi. 


g^^ 

M 

^'•■^r.^^.:      '■^^'^. 

■  ♦ 

B.  Extreme  southwestern  ridge  of  Santa  Catalinas  viewed  from  the  north.  In  foreground  is  the  bed  of  the  Canada 
del  Oro,  with  individuals  of  Hymenoclea  monogyra  and  a  marginal  fringe  of  Prosopis  velutina  and  Chil- 
opsis  linearis. 


SHREVE 


Plate  3 


A.  Typical  L(»\v  I^ajada,  with  pure  .stand  of  Cofillra  trifhnlatn  and  sunmiiT  carijct  of  Boiilvloua  aristidoides. 


B.   Looking  southwest  from  Upper  Bajada  near  mouth  of  Soldiii-  (  anon.     In  the  arro\  u  in  iMrc^im 

Carnegiea  gigantea,  Parkinsonia  microphylla,  Acacia  greggii,  and  Fouquicria  ■•iidcndcns.     Di.stant 
hills  are  relict  toes  of  ancient  bajadas. 


SHREVE 


Plate  4 


SHREVE 


Plate  5 


A.  Desert  Slopes  neai  mouth  of  Pinia  Cdiion.    In  foiegiound,  from  the  left  aie  FcuUn-sonia  mi 
Simmondbia  californica,  Carnegiea  gigantea,  Opunlia  loumeyi,  and  Lycivm  berlandieri. 


,phyUn 


D(MI 

iduttna,  cUkI 
engdmanni. 


Mom  I '■I 


M(iiiit\     i^  A.     Again.st  the  sky  arc  Fonqnicria   .splendent-,  Proxoph: 
pdlhilii    u\  foieground  Spliirralrea  pedata,  Lippia  wrvjhtii,  and  Opnntm 


SHREVE 


Plate  6 


A;  5 

-     \ 

V 

m 

i|ij^ 

a^r^"; 

S,'} 

C2^p 

^ 

^■.■■.id:#f^:f'^i>-^ 

fe-,:-v  ■  -'^^ 

% 

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p 

.  .^^Pi 

'A  «^ldSl<H».<'BK<£%3lQ 

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^   .  r          '    •"'  ' 

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%: 

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J^Sis: 

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;.  ^-cMiMm 

■^^^fe 

^^^'^^f  ^-   "■  t 

idl^B^ 

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^^'"^^£4^ 

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^^.  ■.'■ 

'^^ 

-L^  '  ^^^:^=*' 

REVE 


Plate  7 


SHREVE 


Plate  8 


SHREVE 


Plate  9 


A.  Looking  northeast  along  Lowest  Slopes  of  the  Lower  Encinal  at  4,300  feet.    At  left  Arctontaphylos  pungens,  at 
right  Quercus  ohlongifolia,  below  it  Dasylirion  ivheeleri,  to  left  of  the  latter  Nolina  microcarpa. 


B.  Looking  su 


itheast  in  Soldier  Canon.     On  right  the  tic(l.-^>  -.lope-,  of  the  Upper  Desert,  on  left  open  Encinal 
of  Quercus  ohlonyifoUa  and  Quliiu)>  aiizomca. 


SHREVE 


Plate  10 


A.  Flood-plain  of  Soldier  Canon  at  4,200  feet.    The  predominant  plant  is  Bacchaiis  mrulkruiihs. 


•^a^y^^y 


lopes  and  Flood-plain  of  Soldier  (  anon  at  4,900  feet.    The  predominant  trees  are  Quercus  emoryi 
and  Juniperus  pachyphlaa,  the  shrubs  Arctostaphyloa  puiigens  and  Garrya  wriyhtii. 


SHREVE 


Plate  11 


A.  Encinal  in  Soldier  Canon  at  5,000  feet,  with  Quercus  emoryi,  Juniperus  pachyphlwa,  Garrijn  u'ri/htii, 
Yucca  marrocarpa,  and  Nolina  microcarpa.    At  lower  left  is  Bouteloua  rothrockii. 


B.  Quercus  oblongifolia  in  Flood-plain  of  Soldier  Canon.   At  left  Quercus  emoryi,  at  right  Nolina  m  icrocarpa. 


SHREVE 


Plate  12 


SHREVE 


Plate  13 


—     ,,,ll  ■    ...  . — ^ — — 

''''^jj§ 

M 

W^^^^^^^^^^^^^^^E-                                   W^^K^^^^^^^^^^^KMb^'i^^'fiX'''  -'^'''y'^Si 

A.   Heavy  (.'arpet  of  Summer  ll.ii.M..  i.-  \ 
var.  subcinercn.  Mumiiilii  /nr/ 


<pars-iflora 


,l„     „  In ,,,!,,  a      ,n,l  Si,h„rnlr,.,   s, 


B.  Agaxe  schottii  in  the  Lower  Encinal.    Extensive  areas  between  4,20U  and  5,400  feet  are 
covered  by  this  plant. 


SHREVE 


Plate  14 


A.  Gymnopteris  hispida  occupying  a  ledge  of  rock  in  the  Lower  Encinal  ai  .j,2Ui)  1< 


B.  Mats  of  Selaginella  sp.  among  rock- 


SHREVE 


Plate  15 


SHREVE 


Plate  16 


SHREVE 


Plate  17 


1 

^L 

r 

^HP'?^/ 

r 

'■p-^'""       y% 

V 

l\ 

*M '             ' 

Pi 

JfaggJ 

f'   ■''^niL^ 

...'.  ^ 

J.., 

1|i|g|^Mc 

fi 

^^^1 

P-. 

t'  "^ 

L    ■js'-    i:  JHB^.      •»» 

^£., 

<:^ 

K, 

1 

SI'  •                i>  4 

t% 

Ht 

9 

K 

W             4Ui 

P^^ 

^ 

.:4j*S|f«F 

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i 

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I^^Ivn^^JB^^I 

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ft 

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cg^ 

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f  .;• 

^'w  '■* 

SHREVE 


Plate  18 


SHREVE 


Plate  19 


SHREVE 


Plate  20 


A.  Flood-plain  in  Bear  Canon  at  6,000  feet,  with  Pinus  arizonica,  Populus  jamesii,  an  open  carpet  of 
summer-active  herbaceous  perennials,  and  Sporobolus  confusus. 


B.  Strcamway  in  Bear  Canon  at  6,000  feet,  with  Pinus  arizonica,  Juglans  rupestris,  and  Vitis  arizonica. 


SHREVE 


Plate  21 


Open  stand  of  Finns  a 


onica,  Finus  chihuahuana,  and  Juniperus  pacftyphlwa  near  floor  of  Bear  Canon  at  G,100 
feet.     In  background  are  rocky  slopes  of  north  wall  of  Bear  Canon. 


SHREVE 


Plate  22 


SHREVE 


Plate  23 


SHREVE 


Plate  24 


SHREVE 


Plate  25 


A.   Looking  toward  south  face  of  Mount  Lemmon  from  crest  of  Marshall  Gulch,  near  sitr  of  s.lioil 
elimatological  station.     Pinus  arizonica  and  scrul)  of  Qucrcu.s  rvticulaln. 


B.   Looking  southwest  into  ALushall  Gulch.     The  open  area  in  the  f 


/'n/iuhix  tnmi/loidcs 


SHREVE 


Plate  26 


SEREVE 


Plate  27 


A.  Typical  heavy  stand  of  Pinus  arizonica  on  a  bench  in  Marshall  Gulch  at  7,800  feet. 


B.  Looking  east  along  south-facing  slope  of  Marshall  Gulch  at  7,700  feet.     The  hunch-gruss 
Muhlenbergia  virescens. 


SHREVE 


Plate  28 


SHREVE 


Plate  29 


r^ 

L    >^* 

»fi 

Liiit.  .. 

',   .t- 

ilitfjifi^^ 

.'f      ' '' 

'■  v.-    ■ 

,#* 

A.   Open  Forest  on  Steep  South  Slopes  of  Alain  Ridge  at  .S,5()()  feet.     The  shrulis  are  Qurrciia  reticulata. 


B.  Stream  and  Narrow  Flood-plain  in  Marshall  Gulch  near  Montane  Garden.    Alnus  acuminata,  Acer 
brachypierum,  and  Abies  concolor. 


SHREVE 


Plate  30 


SHREVE 


Plate  31 


SHREVE 


Plate  32 


A.   Looking  northwest  from  main  ridge  toward  Samaniego  Kidge. 


B.   Looking  east  along  main  ridge  of  Santa  Catalinas  from  a  point  east  of  Mount  Lennnon  at  about  S,600  feet. 


SHREVE 


Plate  33 


A.  Open  Forest  on    summit   of   Mount  Lemmon,  at  9,000  feet,  with  good  reproduction  of  Pinus  arizonica  and 

a  close  stand  of  Dugaldia  hoopesii. 


i,(    J'lijiiilns     h'  iniiliiiilrs    oil    summit    of    Mmmmi      I  ..nnni  .n     a 

Fttrid  aquilina  var.  piibescens  and  floweiuig  plant*  ul   lit 


ground    are 


SHREVE 


Plate  34 


SHREVE 


Plate  35 


An  alluvial  flat  in  Fir  Forest  on  north  slopes  of  xMount  Lennnon  at  8,(300  feet.     Pinus  arizonica,  Abies  concolor, 

and  Populus  tremuloides. 


SHREVE 


Plate  36 


A.  SaiiiH  Catalinas  viewed  from  north,  showing  grassy  plains  at  elevation  of  4,l.'()U  feet  in   the  vi 
Oracle.    At  right  Prosopis  velutina,  at  left  Yucca  alata. 


B.  At  north  l.aseof 


ilinas  looking  toward  San  Pedro  River,  at  4,500  feet.    At  i  i 
and  Nolina  microcarpa,  at  left  Yucca  alata. 


%  C.  StaU  CoOtgl 


